/////////////////////////////////////////////////////////////////////////////// version="$Id: homolog.lib,v 1.15.2.2 2002/10/07 14:02:31 lossen Exp $"; category="Commutative Algebra"; info=" LIBRARY: homolog.lib Procedures for Homological Algebra AUTHORS: Gert-Martin Greuel, greuel@mathematik.uni-kl.de, @* Bernd Martin, martin@math.tu-cottbus.de @* Christoph Lossen, lossen@mathematik.uni-kl.de PROCEDURES: cup(M); cup: Ext^1(M',M') x Ext^1() --> Ext^2() cupproduct(M,N,P,p,q); cup: Ext^p(M',N') x Ext^q(N',P') --> Ext^p+q(M',P') depth(I,M); depth(I,M'), I ideal, M module, M'=coker(M) Ext_R(k,M); Ext^k(M',R), M module, R basering, M'=coker(M) Ext(k,M,N); Ext^k(M',N'), M,N modules, M'=coker(M), N'=coker(N) fitting(M,n); n-th Fitting ideal of M'=coker(M), M module, n int flatteningStrat(M); Flattening stratification of M'=coker(M), M module Hom(M,N); Hom(M',N'), M,N modules, M'=coker(M), N'=coker(N) homology(A,B,M,N); ker(B)/im(A), homology of complex R^k--A->M'--B->N' isCM(M); test if coker(M) is Cohen-Macaulay, M module isFlat(M); test if coker(M) is flat, M module isLocallyFree(M,r); test if coker(M) is locally free of constant rank r isReg(I,M); test if I is coker(M)-sequence, I ideal, M module kernel(A,M,N); ker(M'--A->N') M,N modules, A matrix kohom(A,k); Hom(R^k,A), A matrix over basering R kontrahom(A,k); Hom(A,R^k), A matrix over basering R KoszulHomology(I,M,n); n-th Koszul homology H_n(I,coker(M)), I=ideal tensorMod(M,N); Tensor product of modules M'=coker(M), N'=coker(N) Tor(k,M,N); Tor_k(M',N'), M,N modules, M'=coker(M), N'=coker(N) "; LIB "general.lib"; LIB "deform.lib"; LIB "matrix.lib"; LIB "poly.lib"; LIB "primdec.lib"; /////////////////////////////////////////////////////////////////////////////// proc cup (module M,list #) "USAGE: cup(M,[,any,any]); M=module COMPUTE: cup-product Ext^1(M',M') x Ext^1(M',M') ---> Ext^2(M',M'), where M':=R^m/M, if M in R^m, R basering (i.e. M':=coker(matrix(M))). @* If called with >= 2 arguments: compute symmetrized cup-product ASSUME: all Ext's are finite dimensional RETURN: - if called with 1 argument: matrix, the columns of the output present the coordinates of b_i&b_j with respect to a kbase of Ext^2, where b_1,b_2,... is a kbase of Ext^1 and & denotes cup product;@* - if called with 2 arguments: matrix, the columns of the output present the coordinates of (1/2)(b_i&b_j + b_j&b_i) with respect to a kbase of Ext^2; - if called with 3 arguments: list, @format L[1] = matrix see above (symmetric case, for >=2 arguments) L[2] = matrix of kbase of Ext^1 L[3] = matrix of kbase of Ext^2 @end format NOTE: printlevel >=1; shows what is going on. printlevel >=2; shows result in another representation. @* For computing cupproduct of M itself, apply proc to syz(M)! EXAMPLE: example cup; shows examples " { //---------- initialization --------------------------------------------------- int i,j,k,f0,f1,f2,f3,e1,e2; module M1,M2,A,B,C,ker,ima,ext1,ext2,ext10,ext20; matrix cup[1][0]; matrix kb1,lift1,kb2,mA,mB,mC; ideal tes1,tes2,null; int p = printlevel-voice+3; // p=printlevel+1 (default: p=1) //----------------------------------------------------------------------------- //take a resolution of M<--F(0)<--- ... <---F(3) //apply Hom(-,M) and compute the Ext's //----------------------------------------------------------------------------- list resM = nres(M,3); M1 = resM[2]; M2 = resM[3]; f0 = nrows(M); f1 = ncols(M); f2 = ncols(M1); f3 = ncols(M2); tes1 = simplify(ideal(M),10); tes2=simplify(ideal(M1),10); if ((tes1[1]*tes2[1]==0) or (tes1[1]==1) or (tes2[1]==1)) { dbprint(p,"// Ext == 0 , hence 'cup' is the zero-map"); return(@cup); } //------ compute Ext^1 -------------------------------------------------------- B = kohom(M,f2); A = kontrahom(M1,f0); C = intersect(A,B); C = reduce(C,std(null));C = simplify(C,10); ker = lift(A,C)+syz(A); ima = kohom(M,f1); ima = ima + kontrahom(M,f0); ext1 = modulo(ker,ima); ext10 = std(ext1); e1 = vdim(ext10); dbprint(p-1,"// vdim (Ext^1) = "+string(e1)); if (e1 < 0) { "// Ext^1 not of finite dimension"; return(cup); } kb1 = kbase(ext10); kb1 = matrix(ker)*kb1; dbprint(p-1,"// kbase of Ext^1(M,M)", "// - the columns present the kbase elements in Hom(F(1),F(0))", "// - F(*) a free resolution of M",kb1); //------ compute the liftings of Ext^1 ---------------------------------------- mC = matrix(A)*kb1; lift1 =lift(B,mC); dbprint(p-1,"// lift kbase of Ext^1:", "// - the columns present liftings of kbase elements into Hom(F(2),F(1))", "// - F(*) a free resolution of M ",lift1); //------ compute Ext^2 ------------------------------------------------------- B = kohom(M,f3); A = kontrahom(M2,f0); C = intersect(A,B); C = reduce(C,std(null));C = simplify(C,10); ker = lift(A,C)+syz(A); ima = kohom(M,f2); ima = ima + kontrahom(M1,f0); ext2 = modulo(ker,ima); ext20= std(ext2); e2 = vdim(ext20); if (e2<0) { "// Ext^2 not of finite dimension"; return(cup); } dbprint(p-1,"// vdim (Ext^2) = "+string(e2)); kb2 = kbase(ext20); kb2 = matrix(ker)*kb2; dbprint(p-1,"// kbase of Ext^2(M,M)", "// - the columns present the kbase elements in Hom(F(2),F(0))", "// - F(*) is a a free resolution of M ",kb2); //------- compute: cup-products of base-elements ----------------------------- for (i=1;i<=e1;i=i+1) { for (j=1;j<=e1;j=j+1) { mA = matrix(ideal(lift1[j]),f1,f2); mB = matrix(ideal(kb1[i]),f0,f1); mC = mB*mA; if (size(#)==0) { //non symmestric mC = matrix(ideal(mC),f0*f2,1); cup= concat(cup,mC); } else //symmetric version { if (j>=i) { if (j>i) { mA = matrix(ideal(lift1[i]),f1,f2); mB = matrix(ideal(kb1[j]),f0,f1); mC = mC+mB*mA;mC=(1/2)*mC; } mC = matrix(ideal(mC),f0*f2,1); cup= concat(cup,mC); } } } } dbprint(p-1,"// matrix of cup-products (in Ext^2)",cup, "////// end level 2 //////"); //------- compute: presentation of base-elements ----------------------------- cup = lift(ker,cup); cup = lift_kbase(cup,ext20); if( p>2 ) { "// the associated matrices of the bilinear mapping 'cup' "; "// corresponding to the kbase elements of Ext^2(M,M) are shown,"; "// i.e. the rows of the final matrix are written as matrix of"; "// a bilinear form on Ext^1 x Ext^1"; matrix BL[e1][e1]; for (k=1;k<=e2;k=k+1) { "//-----component "+string(k)+":"; for (i=1;i<=e1;i=i+1) { for (j=1;j<=e1;j=j+1) { if (size(#)==0) { BL[i,j]=cup[k,j+e1*(i-1)]; } else { if (i<=j) { BL[i,j]=cup[k,j+e1*(i-1)-binomial(i,2)]; BL[j,i]=BL[i,j]; } } } } print(BL); } "////// end level 3 //////"; } if (size(#)>2) { return(cup,kb1,kb2);} else {return(cup);} } example {"EXAMPLE"; echo=2; int p = printlevel; ring rr = 32003,(x,y,z),(dp,C); ideal I = x4+y3+z2; qring o = std(I); module M = [x,y,0,z],[y2,-x3,z,0],[z,0,-y,-x3],[0,z,x,-y2]; print(cup(M)); print(cup(M,1)); // 2nd EXAMPLE (shows what is going on) printlevel = 3; ring r = 0,(x,y),(dp,C); ideal i = x2-y3; qring q = std(i); module M = [-x,y],[-y2,x]; print(cup(M)); printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc cupproduct (module M,N,P,int p,q,list #) "USAGE: cupproduct(M,N,P,p,q[,any]); M,N,P modules, p,q integers COMPUTE: cup-product Ext^p(M',N') x Ext^q(N',P') ---> Ext^p+q(M',P'), where M':=R^m/M, if M in R^m, R basering (i.e. M':=coker(matrix(M))) ASSUME: all Ext's are of finite dimension RETURN: - if called with 5 arguments: matrix of the associated linear map Ext^p (tensor) Ext^q --> Ext^p+q, i.e. the columns of present the coordinates of the cup products (b_i & c_j) with respect to a kbase of Ext^p+q (b_i resp. c_j are the choosen bases of Ext^p, resp. Ext^q).@* - if called with 6 arguments: list L, @format L[1] = matrix (see above) L[2] = matrix of kbase of Ext^p(M',N') L[3] = matrix of kbase of Ext^q(N',P') L[4] = matrix of kbase of Ext^p+q(N',P') @end format NOTE: printlevel >=1; shows what is going on. printlevel >=2; shows the result in another representation.@* For computing the cupproduct of M,N itself, apply proc to syz(M), syz(N)! EXAMPLE: example cupproduct; shows examples " { //---------- initialization --------------------------------------------------- int e1,e2,e3,i,j,k,f0,f1,f2; module M1,M2,N1,N2,P1,P2,A,B,C,ker,ima,extMN,extMN0,extMP, extMP0,extNP,extNP0; matrix cup[1][0]; matrix kbMN,kbMP,kbNP,lift1,mA,mB,mC; ideal test1,test2,null; int pp = printlevel-voice+3; // pp=printlevel+1 (default: p=1) //----------------------------------------------------------------------------- //compute resolutions of M and N // M<--F(0)<--- ... <---F(p+q+1) // N<--G(0)<--- ... <---G(q+1) //----------------------------------------------------------------------------- list resM = nres(M,p+q+1); M1 = resM[p]; M2 = resM[p+1]; list resN = nres(N,q+1); N1 = resN[q]; N2 = resN[q+1]; P1 = resM[p+q]; P2 = resM[p+q+1]; //-------test: Ext==0?--------------------------------------------------------- test1 = simplify(ideal(M1),10); test2 = simplify(ideal(N),10); if (test1[1]==0) { dbprint(pp,"//Ext(M,N)=0");return(cup); } test1 = simplify(ideal(N1),10); test2 = simplify(ideal(P),10); if (test1[1]==0) { dbprint(pp,"//Ext(N,P)=0");return(cup); } test1 = simplify(ideal(P1),10); if (test1[1]==0) { dbprint(pp,"//Ext(M,P)=0");return(cup); } //------ compute kbases of Ext's --------------------------------------------- //------ Ext(M,N) test1 = simplify(ideal(M2),10); if (test1[1]==0) { ker = freemodule(ncols(M1)*nrows(N));} else { A = kontrahom(M2,nrows(N)); B = kohom(N,ncols(M2)); C = intersect(A,B); C = reduce(C,std(ideal(0)));C=simplify(C,10); ker = lift(A,C)+syz(A); } ima = kohom(N,ncols(M1)); A = kontrahom(M1,nrows(N)); ima = ima+A; extMN = modulo(ker,ima); extMN0= std(extMN); e1 = vdim(extMN0); dbprint(pp-1,"// vdim Ext(M,N) = "+string(e1)); if (e1 < 0) { "// Ext(M,N) not of finite dimension"; return(cup); } kbMN = kbase(extMN0); kbMN = matrix(ker)*kbMN; dbprint(pp-1,"// kbase of Ext^p(M,N)", "// - the columns present the kbase elements in Hom(F(p),G(0))", "// - F(*),G(*) are free resolutions of M and N",kbMN); //------- Ext(N,P) test1 = simplify(ideal(N2),10); if (test1[1]==0) { ker = freemodule(ncols(N1)*nrows(P)); } else { A = kontrahom(N2,nrows(P)); B = kohom(P,ncols(N2)); C = intersect(A,B); C = reduce(C,std(ideal(0)));C=simplify(C,10); ker = lift(A,C)+syz(A); } ima = kohom(P,ncols(N1)); A = kontrahom(N1,nrows(P)); ima = ima+A; extNP = modulo(ker,ima); extNP0= std(extNP); e2 = vdim(extNP0); dbprint(pp-1,"// vdim Ext(N,P) = "+string(e2)); if (e2 < 0) { "// Ext(N,P) not of finite dimension"; return(cup); } kbNP = kbase(extNP0); kbNP = matrix(ker)*kbNP; dbprint(pp-1,"// kbase of Ext(N,P):",kbNP, "// kbase of Ext^q(N,P)", "// - the columns present the kbase elements in Hom(G(q),H(0))", "// - G(*),H(*) are free resolutions of N and P",kbNP); //------ Ext(M,P) test1 = simplify(ideal(P2),10); if (test1[1]==0) { ker = freemodule(ncols(P1)*nrows(P)); } else { A = kontrahom(P2,nrows(P)); B = kohom(P,ncols(P2)); C = intersect(A,B); C = reduce(C,std(ideal(0)));C=simplify(C,10); ker = lift(A,C)+syz(A); } ima = kohom(P,ncols(P1)); A = kontrahom(P1,nrows(P)); ima = ima+A; extMP = modulo(ker,ima); extMP0= std(extMP); e3 = vdim(extMP0); dbprint(pp-1,"// vdim Ext(M,P) = "+string(e3)); if (e3 < 0) { "// Ext(M,P) not of finite dimension"; return(cup); } kbMP = kbase(extMP0); kbMP = matrix(ker)*kbMP; dbprint(pp-1,"// kbase of Ext^p+q(M,P)", "// - the columns present the kbase elements in Hom(F(p+q),H(0))", "// - F(*),H(*) are free resolutions of M and P",kbMP); //----- lift kbase of Ext(M,N) ------------------------------------------------ lift1 = kbMN; for (i=1;i<=q;i=i+1) { mA = kontrahom(resM[p+i],nrows(resN[i])); mB = kohom(resN[i],ncols(resM[p+i])); lift1 = lift(mB,mA*lift1); } dbprint(pp-1,"// lifting of kbase of Ext^p(M,N)", "// - the columns present liftings of kbase elements"+ " in Hom(F(p+q),G(q))",lift1); //------- compute: cup-products of base-elements ----------------------------- f0 = nrows(P); f1 = ncols(N1); f2 = ncols(resM[p+q]); for (i=1;i<=e1;i=i+1) { for (j=1;j<=e2;j=j+1) { mA = matrix(ideal(lift1[j]),f1,f2); mB = matrix(ideal(kbMP[i]),f0,f1); mC = mB*mA; mC = matrix(ideal(mC),f0*f2,1); cup= concat(cup,mC); } } dbprint(pp-1,"// matrix of cup-products (in Ext^p+q)",cup, "////// end level 2 //////"); //------- compute: presentation of base-elements ----------------------------- cup = lift(ker,cup); cup = lift_kbase(cup,extMP0); //------- special output ------------------------------------------------------ if (pp>2) { "// the associated matrices of the bilinear mapping 'cup' "; "// corresponding to the kbase elements of Ext^p+q(M,P) are shown,"; "// i.e. the rows of the final matrix are written as matrix of"; "// a bilinear form on Ext^p x Ext^q"; matrix BL[e1][e2]; for (k=1;k<=e3;k=k+1) { "//----component "+string(k)+":"; for (i=1;i<=e1;i=i+1) { for (j=1;j<=e2;j=j+1) { BL[i,j]=cup[k,j+e1*(i-1)]; } } print(BL); } "////// end level 3 //////"; } if (size(#)) { return(cup,kbMN,kbNP,kbMP);} else { return(cup); } } example {"EXAMPLE"; echo=2; int p = printlevel; ring rr = 32003,(x,y,z),(dp,C); ideal I = x4+y3+z2; qring o = std(I); module M = [x,y,0,z],[y2,-x3,z,0],[z,0,-y,-x3],[0,z,x,-y2]; print(cupproduct(M,M,M,1,3)); printlevel = 3; list l = (cupproduct(M,M,M,1,3,"any")); show(l[1]);show(l[2]); printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc Ext_R (intvec v, module M, list #) "USAGE: Ext_R(v,M[,p]); v int resp. intvec , M module, p int COMPUTE: A presentation of Ext^k(M',R); for k=v[1],v[2],..., M'=coker(M). Let @example 0 <-- M' <-- F0 <-M-- F1 <-- F2 <-- ... @end example be a free resolution of M'. If @example 0 --> F0* -A1-> F1* -A2-> F2* -A3-> ... @end example is the dual sequence, Fi*=Hom(Fi,R), then Ext^k = ker(Ak+1)/im(Ak) is presented as in the following exact sequences: @example R^p --syz(Ak+1)-> Fk* ---Ak+1----> Fk+1* , R^q ----Ext^k---> R^p --syz(Ak+1)-> Fk*/im(Ak). @end example Hence, Ext^k=modulo(syz(Ak+1),Ak) presents Ext^k(M',R). RETURN: - module Ext, a presentation of Ext^k(M',R) if v is of type int@* - a list of Ext^k (k=v[1],v[2],...) if v is of type intvec.@* - In case of a third argument of type int return a list l: @format l[1] = module Ext^k resp. list of Ext^k l[2] = SB of Ext^k resp. list of SB of Ext^k l[3] = matrix resp. list of matrices, each representing a kbase of Ext^k (if finite dimensional) @end format DISPLAY: printlevel >=0: (affine) dimension of Ext^k for each k (default) printlevel >=1: Ak, Ak+1 and kbase of Ext^k in Fk* NOTE: In order to compute Ext^k(M,R) use the command Ext_R(k,syz(M)); or the 2 commands: list L=mres(M,2); Ext_R(k,L[2]); EXAMPLE: example Ext_R; shows an example " { // In case M is known to be a SB, set attrib(M,"isSB",1); in order to // avoid unnecessary SB computations //------------ initialisation ------------------------------------------------- module m1,m2,ret,ret0; matrix ker,kb; list L1,L2,L3,L,resl,K; int k,max,ii,t1,t2,di; int s = size(v); intvec v1 = sort(v)[1]; max = v1[s]; // the maximum integer occurring in intvec v int p = printlevel-voice+3; // p=printlevel+1 (default: p=1) // --------------- Variante mit sres for( ii=1; ii<=size(#); ii++ ) { t2=1; // return a list if t2=1 if( typeof(#[ii])=="string" ) { if ( #[ii]=="sres" ) { t1=1; t2=0; } // use sres instead of mres if t1=1 } } //----------------- compute resolution of coker(M) ---------------------------- if( max<0 ) { dbprint(p,"// Ext^i=0 for i<0!"); return([1]); } if( t1==1 ) { if( attrib(M,"isSB")==0 ) { M=std(M); } resl = sres(M,max+1); } else { resl = mres(M,max+1); } for( ii=1; ii<=s; ii++ ) { //----------------- apply Hom(_,R) at k-th place ----------------------------- k=v[ii]; if( k<0 ) // Ext^k=0 for negative k { dbprint(p-1,"// Ext^i=0 for i<0!"); ret = gen(1); ret0 = std(ret); L1[ii] = ret; L2[ii] = ret0; L3[ii] = matrix(kbase(ret0)); di=dim(ret0); dbprint(p,"// dimension of Ext^"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Ext^"+string(k)+": "+string(vdim(ret0))); } dbprint(p,""); } else { m2 = transpose(resl[k+1]); if( k==0 ) { m1=0*gen(nrows(m2)); } else { m1 = transpose(resl[k]); } //----------------- presentation of ker(m2)/im(m1) ---------------------------- ker = syz(m2); ret = modulo(ker,m1); dbprint(p-1,"// Computing Ext^"+string(k)+":", "// Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of M,", "// then F"+string(k)+"*-->F"+string(k+1)+"* is given by:",m2, "// and F"+string(k-1)+"*-->F"+string(k)+"* is given by:",m1,""); ret0 = std(ret); di=dim(ret0); dbprint(p,"// dimension of Ext^"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Ext^"+string(k)+": "+string(vdim(ret0))); } dbprint(p,""); if( t2 ) { if( vdim(ret0)>=0 ) { kb = kbase(ret0); if ( size(ker)!=0 ) { kb = matrix(ker)*kb; } dbprint(p-1, "// columns of matrix are kbase of Ext^"+string(k)+" in F" +string(k)+"*:",kb,""); L3[ii] = kb; } L2[ii] = ret0; } L1[ii] = ret; } } if( t2 ) { if( s>1 ) { L = L1,L2,L3; return(L); } else { L = ret,ret0,kb; return(L); } } else { if( s>1 ) { return(L1); } else { return(ret); } } } example {"EXAMPLE:"; echo=2; int p = printlevel; printlevel = 1; ring r = 0,(x,y,z),dp; ideal i = x2y,y2z,z3x; module E = Ext_R(1,i); //computes Ext^1(r/i,r) is_zero(E); qring R = std(x2+yz); intvec v = 0,2; printlevel = 2; //shows what is going on ideal i = x,y,z; //computes Ext^i(r/(x,y,z),r/(x2+yz)), i=0,2 list L = Ext_R(v,i,1); //over the qring R=r/(x2+yz), std and kbase printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc Ext (intvec v, module M, module N, list #) "USAGE: Ext(v,M,N[,any]); v int resp. intvec, M,N modules COMPUTE: A presentation of Ext^k(M',N'); for k=v[1],v[2],... where M'=coker(M) and N'=coker(N). Let @example 0 <-- M' <-- F0 <-M-- F1 <-- F2 <--... , 0 <-- N' <-- G0 <--N- G1 @end example be a free resolution of M', resp. a presentation of N'. Consider the commutative diagram @example 0 0 0 |^ |^ |^ --> Hom(Fk-1,N') -Ak-> Hom(Fk,N') -Ak+1-> Hom(Fk+1,N') |^ |^ |^ --> Hom(Fk-1,G0) -Ak-> Hom(Fk,G0) -Ak+1-> Hom(Fk+1,G0) |^ |^ |C |B Hom(Fk,G1) ------> Hom(Fk+1,G1) (Ak,Ak+1 induced by M and B,C induced by N). @end example Let K=modulo(Ak+1,B), J=module(Ak)+module(C) and Ext=modulo(K,J), then we have exact sequences @example R^p --K-> Hom(Fk,G0) --Ak+1-> Hom(Fk+1,G0)/im(B), R^q -Ext-> R^p --K-> Hom(Fk,G0)/(im(Ak)+im(C)). @end example Hence, Ext presents Ext^k(M',N'). RETURN: - module Ext, a presentation of Ext^k(M',N') if v is of type int@* - a list of Ext^k (k=v[1],v[2],...) if v is of type intvec.@* - In case of a third argument of any type return a list l: @format l[1] = module Ext/list of Ext^k l[2] = SB of Ext/list of SB of Ext^k l[3] = matrix/list of matrices, each representing a kbase of Ext^k (if finite dimensional) @end format DISPLAY: printlevel >=0: dimension, vdim of Ext^k for each k (default). @* printlevel >=1: matrices Ak, Ak+1 and kbase of Ext^k in Hom(Fk,G0) (if finite dimensional) NOTE: In order to compute Ext^k(M,N) use the command Ext(k,syz(M),syz(N)); or: list P=mres(M,2); list Q=mres(N,2); Ext(k,P[2],Q[2]); EXAMPLE: example Ext; shows an example " { //---------- initialisation --------------------------------------------------- int k,max,ii,l,row,col,di; module A,B,C,D,M1,M2,N1,ker,imag,extMN,extMN0; matrix kb; list L1,L2,L3,L,resM,K; ideal test1; intmat Be; int s = size(v); intvec v1 = sort(v)[1]; max = v1[s]; // the maximum integer occurring in intvec v int p = printlevel-voice+3; // p=printlevel+1 (default: p=1) //---------- test: coker(N)=basering, coker(N)=0 ? ---------------------------- if( max<0 ) { dbprint(p,"// Ext^i=0 for i<0!"); return([1]); } N1 = std(N); if( size(N1)==0 ) //coker(N)=basering, in this case proc Ext_R is faster { printlevel=printlevel+1; if( size(#)==0 ) { def E = Ext_R(v,M); printlevel=printlevel-1; return(E); } else { def E = Ext_R(v,M,#[1]); printlevel=printlevel-1; return(E); } } if( dim(N1)==-1 ) //coker(N)=0, all Ext-groups are 0 { dbprint(p-1,"2nd module presents 0, hence Ext^k=0, for all k"); for( ii=1; ii<=s; ii++ ) { k=v[ii]; extMN = gen(1); extMN0 = std(extMN); L1[ii] = extMN; L2[ii] = extMN0; L3[ii] = matrix(kbase(extMN0)); di=dim(extMN0); dbprint(p,"// dimension of Ext^"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Ext^"+string(k)+": "+string(vdim(extMN0))); } dbprint(p,""); } } else { if( size(N1) < size(N) ) { N=N1;} row = nrows(N); //---------- resolution of M ------------------------------------------------- resM = mres(M,max+1); for( ii=1; ii<=s; ii++ ) { k=v[ii]; if( k<0 ) // Ext^k is 0 for negative k { dbprint(p-1,"// Ext^k=0 for k<0!"); extMN = gen(1); extMN0 = std(extMN); L1[ii] = extMN; L2[ii] = extMN0; L3[ii] = matrix(kbase(extMN0)); di=dim(extMN0); dbprint(p,"// dimension of Ext^"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Ext^"+string(k)+": " +string(vdim(extMN0))); } dbprint(p,""); } else { M2 = resM[k+1]; if( k==0 ) { M1=0*gen(nrows(M2)); } else { M1 = resM[k]; } col = ncols(M1); D = kohom(N,col); //---------- computing homology ---------------------------------------------- imag = kontrahom(M1,row); A = kontrahom(M2,row); B = kohom(N,ncols(M2)); ker = modulo(A,B); imag = imag,D; extMN = modulo(ker,imag); dbprint(p-1,"// Computing Ext^"+string(k)+ " (help Ext; gives an explanation):", "// Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),", "// and 0<--coker(N)<--G0<--G1 a presentation of coker(N),", "// then Hom(F"+string(k)+",G0)-->Hom(F"+string(k+1)+ ",G0) is given by:",A, "// and Hom(F"+string(k-1)+",G0) + Hom(F"+string(k)+",G1)-->Hom(F" +string(k)+",G0) is given by:",imag,""); extMN0 = std(extMN); di=dim(extMN0); dbprint(p,"// dimension of Ext^"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Ext^"+string(k)+": " +string(vdim(extMN0))); } dbprint(p,""); //---------- more information ------------------------------------------------- if( size(#)>0 ) { if( vdim(extMN0) >= 0 ) { kb = kbase(extMN0); if ( size(ker)!=0) { kb = matrix(ker)*kb; } dbprint(p-1,"// columns of matrix are kbase of Ext^"+ string(k)+" in Hom(F"+string(k)+",G0)",kb,""); if( p>0 ) { for (l=1;l<=ncols(kb);l=l+1) { "// element",l,"of kbase of Ext^"+string(k)+" in Hom(F"+string(k)+",G0)"; "// as matrix: F"+string(k)+"-->G0"; print(matrix(ideal(kb[l]),row,col)); } ""; } L3[ii] = matrix(kb); } L2[ii] = extMN0; } L1[ii] = extMN; } } } if( size(#) ) { if( s>1 ) { L = L1,L2,L3; return(L); } else { L = extMN,extMN0,matrix(kb); return(L); } } else { if( s>1 ) { return(L1); } else { return(extMN); } } } example {"EXAMPLE:"; echo=2; int p = printlevel; printlevel = 1; ring r = 0,(x,y),dp; ideal i = x2-y3; ideal j = x2-y5; list E = Ext(0..2,i,j); // Ext^k(r/i,r/j) for k=0,1,2 over r qring R = std(i); ideal j = fetch(r,j); module M = [-x,y],[-y2,x]; printlevel = 2; module E1 = Ext(1,M,j); // Ext^1(R^2/M,R/j) over R=r/i list l = Ext(4,M,M,1); // Ext^4(R^2/M,R^2/M) over R=r/i printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc Hom (module M, module N, list #) "USAGE: Hom(M,N,[any]); M,N=modules COMPUTE: A presentation of Hom(M',N'), M'=coker(M), N'=coker(N) as follows: let @example F1 --M-> F0 -->M' --> 0, G1 --N-> G0 --> N' --> 0 @end example be presentations of M' and N'. Consider @example 0 0 |^ |^ 0 --> Hom(M',N') ----> Hom(F0,N') ----> Hom(F1,N') |^ |^ (A: induced by M) Hom(F0,G0) --A-> Hom(F1,G0) |^ |^ (B,C:induced by N) |C |B Hom(F0,G1) ----> Hom(F1,G1) @end example Let D=modulo(A,B) and Hom=modulo(D,C), then we have exact sequences @example R^p --D-> Hom(F0,G0) --A-> Hom(F1,G0)/im(B), R^q -Hom-> R^p --D-> Hom(F0,G0)/im(C) --A-> Hom(F1,G0)/im(B). @end example Hence Hom presents Hom(M',N') RETURN: module Hom, a presentation of Hom(M',N'), resp., in case of 3 arguments, a list l (of size <=3): @format - l[1] = Hom - l[2] = SB of Hom - l[3] = kbase of coker(Hom) (if finite dimensional, not 0), represented by elements in Hom(F0,G0) via mapping D @end format DISPLAY: printlevel >=0: (affine) dimension of Hom (default) @* printlevel >=1: D and C and kbase of coker(Hom) in Hom(F0,G0) @* printlevel >=2: elements of kbase of coker(Hom) as matrix :F0-->G0 NOTE: DISPLAY is as described only for a direct call of 'Hom'. Calling 'Hom' from another proc has the same effect as decreasing printlevel by 1. EXAMPLE: example Hom; shows examples " { //---------- initialisation --------------------------------------------------- int l,p,di; matrix kb; module A,B,C,D,homMN,homMN0; list L; //---------- computation of Hom ----------------------------------------------- B = kohom(N,ncols(M)); A = kontrahom(M,nrows(N)); C = kohom(N,nrows(M)); D = modulo(A,B); homMN = modulo(D,C); homMN0= std(homMN); p = printlevel-voice+3; // p=printlevel+1 (default: p=1) di= dim(homMN0); dbprint(p,"// dimension of Hom: "+string(di)); if (di==0) { dbprint(p,"// vdim of Hom: "+string(vdim(homMN0))); } dbprint(p,""); dbprint(p-1, "// given F1 --M-> F0 -->M'--> 0 and G1 --N-> G0 -->N'--> 0,", "// show D = ker( Hom(F0,G0) --> Hom(F1,G0)/im(Hom(F1,G1)->Hom(F1,G0)) )",D, "// show C = im ( Hom(F0,G1) --> Hom(F0,G0) )",C,""); //---------- extra output if size(#)>0 ---------------------------------------- if( size(#)>0 ) { if( vdim(homMN0)>0 ) { kb = kbase(homMN0); kb = matrix(D)*kb; if( p>2 ) { for (l=1;l<=ncols(kb);l=l+1) { "// element",l,"of kbase of Hom in Hom(F0,G0) as matrix: F0-->G0:"; print(matrix(ideal(kb[l]),nrows(N),nrows(M))); } } else { dbprint(p-1,"// columns of matrix are kbase of Hom in Hom(F0,G0)", kb); } L=homMN,homMN0,kb; return(L); } L=homMN,homMN0; return(L); } return(homMN); } example {"EXAMPLE:"; echo = 2; int p = printlevel; printlevel= 1; //in 'example proc' printlevel has to be increased by 1 ring r = 0,(x,y),dp; ideal i = x2-y3,xy; qring q = std(i); ideal i = fetch(r,i); module M = [-x,y],[-y2,x],[x3]; module H = Hom(M,i); print(H); printlevel= 2; list L = Hom(M,i,1);""; printlevel=1; ring s = 3,(x,y,z),(c,dp); ideal i = jacob(ideal(x2+y5+z4)); qring rq=std(i); matrix M[2][2]=xy,x3,5y,4z,x2; matrix N[3][2]=x2,x,y3,3xz,x2z,z; print(M); print(N); list l=Hom(M,N,1); printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc homology (matrix A,matrix B,module M,module N,list #) "USAGE: homology(A,B,M,N); COMPUTE: Let M and N be submodules of R^m and R^n presenting M'=R^m/M, N'=R^n/N (R=basering) and let A,B matrices inducing maps @example R^k --A--> R^m --B--> R^n. @end example Compute a presentation of the module @example ker(B)/im(A) := ker(M'/im(A) --B--> N'/im(BM)+im(BA)). @end example If B induces a map M'-->N' (i.e BM=0) and if im(A) is contained in ker(B) (that is, BA=0) then ker(B)/im(A) is the homology of the complex @example R^k--A-->M'--B-->N'. @end example RETURN: module H, a presentation of ker(B)/im(A). NOTE: homology returns a free module of rank m if ker(B)=im(A). EXAMPLE: example homology; shows examples " { module ker,ima; ker = modulo(B,N); ima = A,M; return(modulo(ker,ima)); } example {"EXAMPLE"; echo=2; ring r; ideal id=maxideal(4); qring qr=std(id); module N=maxideal(3)*freemodule(2); module M=maxideal(2)*freemodule(2); module B=[2x,0],[x,y],[z2,y]; module A=M; module H=homology(A,B,M,N); H=std(H); // dimension of homology: dim(H); // vector space dimension: vdim(H); ring s=0,x,ds; qring qs=std(x4); module A=[x]; module B=A; module M=[x3]; module N=M; homology(A,B,M,N); } ////////////////////////////////////////////////////////////////////////////// proc kernel (matrix A,module M,module N) "USAGE: kernel(A,M,N); COMPUTE: Let M and N be submodules of R^m and R^n, presenting M'=R^m/M, N'=R^n/N (R=basering), and let A:R^m-->R^n be a matrix inducing a map A':M'-->N'. Then kernel(A,M,N); computes a presentation K of ker(A') as in the commutative diagram: @example ker(A') ---> M' --A'--> N' |^ |^ |^ | | | R^r ---> R^m --A--> R^n |^ |^ |^ |K |M |N | | | R^s ---> R^p -----> R^q @end example RETURN: module K, a presentation of ker(A':coker(M)->coker(N)). EXAMPLE: example kernel; shows examples. " { module M1 = modulo(A,N); return(modulo(M1,M)); } example {"EXAMPLE"; echo=2; ring r; module N=[2x,x],[0,y]; module M=maxideal(1)*freemodule(2); matrix A[2][2]=2x,0,x,y,z2,y; module K=kernel(A,M,N); // dimension of kernel: dim(std(K)); // vector space dimension of kernel: vdim(std(K)); print(K); } /////////////////////////////////////////////////////////////////////////////// proc kohom (matrix M, int j) "USAGE: kohom(A,k); A=matrix, k=integer RETURN: matrix Hom(R^k,A), i.e. let A be a matrix defining a map F1-->F2 of free R-modules, then the matrix of Hom(R^k,F1)-->Hom(R^k,F2) is computed (R=basering). EXAMPLE: example kohom; shows an example. " { if (j==1) { return(M);} if (j>1) { return(tensor(M,diag(1,j))); } else { return(0);} } example {"EXAMPLE:"; echo=2; ring r; matrix n[2][3]=x,y,5,z,77,33; print(kohom(n,3)); } /////////////////////////////////////////////////////////////////////////////// proc kontrahom (matrix M, int j) "USAGE: kontrahom(A,k); A=matrix, k=integer RETURN: matrix Hom(A,R^k), i.e. let A be a matrix defining a map F1-->F2 of free R-modules, then the matrix of Hom(F2,R^k)-->Hom(F1,R^k) is computed (R=basering). EXAMPLE: example kontrahom; shows an example. " { if (j==1) { return(transpose(M));} if (j>1) { return(transpose(tensor(diag(1,j),M)));} else { return(0);} } example {"EXAMPLE:"; echo=2; ring r; matrix n[2][3]=x,y,5,z,77,33; print(kontrahom(n,3)); } /////////////////////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////////////////// proc tensorMod(module Phi, module Psi) "USAGE: tensorMod(M,N); M,N modules COMPUTE: presentation matrix A of the tensor product T of the modules M'=coker(M), N'=coker(N): if matrix(M) defines a map M: R^r-->R^s and matrix(N) defines a map N: R^p-->R^q, then A defines a presentation @example R^(sp+rq) --A-> R^(sq) --> T --> 0 . @end example RETURN: matrix A satisfying coker(A) = tensorprod(coker(M),coker(N)) . EXAMPLE: example tensorMod; shows an example. " { int s=nrows(Phi); int q=nrows(Psi); matrix A=tensor(unitmat(s),Psi); matrix B=tensor(Phi,unitmat(q)); matrix R=concat(A,B); return(R); } example {"EXAMPLE:"; echo=2; ring A=0,(x,y,z),dp; matrix M[3][3]=1,2,3,4,5,6,7,8,9; matrix N[2][2]=x,y,0,z; print(M); print(N); print(tensorMod(M,N)); } /////////////////////////////////////////////////////////////////////////////// proc Tor(intvec v, module M, module N, list #) "USAGE: Tor(v,M,N[,any]); v int resp. intvec, M,N modules COMPUTE: a presentation of Tor_k(M',N'), for k=v[1],v[2],... , where M'=coker(M) and N'=coker(N): let @example 0 <-- M' <-- G0 <-M-- G1 0 <-- N' <-- F0 <--N- F1 <-- F2 <--... @end example be a presentation of M', resp. a free resolution of N', and consider the commutative diagram @example 0 0 0 |^ |^ |^ Tensor(M',Fk+1) -Ak+1-> Tensor(M',Fk) -Ak-> Tensor(M',Fk-1) |^ |^ |^ Tensor(G0,Fk+1) -Ak+1-> Tensor(G0,Fk) -Ak-> Tensor(G0,Fk-1) |^ |^ |C |B Tensor(G1,Fk) ----> Tensor(G1,Fk-1) (Ak,Ak+1 induced by N and B,C induced by M). @end example Let K=modulo(Ak,B), J=module(C)+module(Ak+1) and Tor=modulo(K,J), then we have exact sequences @example R^p --K-> Tensor(G0,Fk) --Ak-> Tensor(G0,Fk-1)/im(B), R^q -Tor-> R^p --K-> Tensor(G0,Fk)/(im(C)+im(Ak+1)). @end example Hence, Tor presents Tor_k(M',N'). RETURN: - if v is of type int: module Tor, a presentation of Tor_k(M',N');@* - if v is of type intvec: a list of Tor_k(M',N') (k=v[1],v[2],...);@* - in case of a third argument of any type: list l with @format l[1] = module Tor/list of Tor_k(M',N'), l[2] = SB of Tor/list of SB of Tor_k(M',N'), l[3] = matrix/list of matrices, each representing a kbase of Tor_k(M',N') (if finite dimensional), or 0. @end format DISPLAY: printlevel >=0: (affine) dimension of Tor_k for each k (default). @* printlevel >=1: matrices Ak, Ak+1 and kbase of Tor_k in Tensor(G0,Fk) (if finite dimensional). NOTE: In order to compute Tor_k(M,N) use the command Tor(k,syz(M),syz(N)); or: list P=mres(M,2); list Q=mres(N,2); Tor(k,P[2],Q[2]); EXAMPLE: example Tor; shows an example { //---------- initialisation --------------------------------------------------- int k,max,ii,l,row,col,di; module A,B,C,D,N1,N2,M1,ker,imag,Im,Im1,Im2,f,torMN,torMN0; matrix kb; list L1,L2,L3,L,resN,K; ideal test1; intmat Be; int s = size(v); intvec v1 = sort(v)[1]; max = v1[s]; // maximum integer occurring in intvec v int p = printlevel-voice+3; // p=printlevel+1 (default: p=1) //---------- test: coker(M)=basering, coker(M)=0 ? ---------------------------- if( max<0 ) { dbprint(p,"// Tor_i=0 for i<0!"); return([1]); } M1 = std(M); if( size(M1)==0 or size(N)==0 ) // coker(M)=basering ==> Tor_i=0 for i>0 { dbprint(p-1,"// one of the modules M',N' is free, hence Tor_i=0 for i<>0"); for( ii=1; ii<=s; ii++ ) { k=v[ii]; if (k==0) { torMN=module(tensorMod(M1,N)); } else { torMN = gen(1); } torMN0 = std(torMN); L1[ii] = torMN; L2[ii] = torMN0; L3[ii] = matrix(kbase(torMN0)); di=dim(torMN0); dbprint(p,"// dimension of Tor_"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Tor_"+string(k)+": " +string(vdim(torMN0))); } dbprint(p,""); } if( size(#) ) { if( s>1 ) { L = L1,L2,L3; return(L); } else { L = torMN,torMN0,L3[1]; return(L); } } else { if( s>1 ) { return(L1); } else { return(torMN); } } } if( dim(M1)==-1 ) // coker(M)=0, all Tor's are 0 { dbprint(p-1,"2nd module presents 0, hence Tor_k=0, for all k"); for( ii=1; ii<=s; ii++ ) { k=v[ii]; torMN = gen(1); torMN0 = std(torMN); L1[ii] = torMN; L2[ii] = torMN0; L3[ii] = matrix(kbase(torMN0)); di=dim(torMN0); dbprint(p,"// dimension of Tor_"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Tor_"+string(k)+": " +string(vdim(torMN0))); } dbprint(p,""); } } else { if( size(M1) < size(M) ) { M=M1;} row = nrows(M); //---------- resolution of N ------------------------------------------------- resN = mres(N,max+1); for( ii=1; ii<=s; ii++ ) { k=v[ii]; if( k<0 ) // Tor_k is 0 for negative k { dbprint(p-1,"// Tor_k=0 for k<0!"); torMN = gen(1); torMN0 = std(torMN); L1[ii] = torMN; L2[ii] = torMN0; L3[ii] = matrix(kbase(torMN0)); di=dim(torMN0); dbprint(p,"// dimension of Tor_"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Tor_"+string(k)+": " +string(vdim(torMN0))); } dbprint(p,""); } else { N2 = resN[k+1]; if( k==0 ) { torMN=module(tensorMod(M,N)); } else { N1 = resN[k]; col = ncols(N1); //---------- computing homology ---------------------------------------------- imag = tensor(unitmat(nrows(N1)),M); f = tensor(matrix(N1),unitmat(row)); Im1 = tensor(unitmat(col),M); Im2 = tensor(matrix(N2),unitmat(row)); ker = modulo(f,imag); Im = Im2,Im1; torMN = modulo(ker,Im); dbprint(p-1,"// Computing Tor_"+string(k)+ " (help Tor; gives an explanation):", "// Let 0 <- coker(M) <- G0 <-M- G1 be the present. of coker(M),", "// and 0 <- coker(N) <- F0 <-N- F1 <- F2 <- ... a resolution of", "// coker(N), then Tensor(G0,F"+string(k)+")-->Tensor(G0,F"+ string(k-1)+") is given by:",f, "// and Tensor(G0,F"+string(k+1)+") + Tensor(G1,F"+string(k)+ ")-->Tensor(G0,F"+string(k)+") is given by:",Im,""); } torMN0 = std(torMN); di=dim(torMN0); dbprint(p,"// dimension of Tor_"+string(k)+": "+string(di)); if (di==0) { dbprint(p,"// vdim of Tor_"+string(k)+": " +string(vdim(torMN0))); } dbprint(p,""); //---------- more information ------------------------------------------------- if( size(#)>0 ) { if( vdim(torMN0) >= 0 ) { kb = kbase(torMN0); if ( size(ker)!=0) { kb = matrix(ker)*kb; } dbprint(p-1,"// columns of matrix are kbase of Tor_"+ string(k)+" in Tensor(G0,F"+string(k)+")",kb,""); L3[ii] = matrix(kb); } L2[ii] = torMN0; } L1[ii] = torMN; } } } if( size(#) ) { if( s>1 ) { L = L1,L2,L3; return(L); } else { L = torMN,torMN0,matrix(kb); return(L); } } else { if( s>1 ) { return(L1); } else { return(torMN); } } } example {"EXAMPLE:"; echo=2; int p = printlevel; printlevel = 1; ring r = 0,(x,y),dp; ideal i = x2,y; ideal j = x; list E = Tor(0..2,i,j); // Tor_k(r/i,r/j) for k=0,1,2 over r qring R = std(i); ideal j = fetch(r,j); module M = [x,0],[0,x]; printlevel = 2; module E1 = Tor(1,M,j); // Tor_1(R^2/M,R/j) over R=r/i list l = Tor(3,M,M,1); // Tor_3(R^2/M,R^2/M) over R=r/i printlevel = p; } /////////////////////////////////////////////////////////////////////////////// proc fitting(module M, int n) "USAGE: fitting (M,n); M module, n int RETURN: ideal, (standard basis of) n-th Fitting ideal of M'=coker(M). EXAMPLE: example fitting; shows an example " { n=nrows(M)-n; if(n<=0){return(ideal(1));} if((n>nrows(M))||(n>ncols(M))){return(ideal(0));} return(std(minor(M,n))); } example {"EXAMPLE:"; echo=2; ring R=0,x(0..4),dp; matrix M[2][4]=x(0),x(1),x(2),x(3),x(1),x(2),x(3),x(4); print(M); fitting(M,-1); fitting(M,0); fitting(M,1); fitting(M,2); } /////////////////////////////////////////////////////////////////////////////// proc isLocallyFree(matrix S, int r) "USAGE: isLocallyFree(M,r); M module, r int RETURN: 1 if M'=coker(M) is locally free of constant rank r;@* 0 if this is not the case. EXAMPLE: example isLocallyFree; shows an example. " { ideal F=fitting(S,r); ideal G=fitting(S,r-1); if((deg(F[1])==0)&&(size(G)==0)){return(1);} return(0); } example {"EXAMPLE:"; echo=2; ring R=0,(x,y,z),dp; matrix M[2][3]; // the presentation matrix M=x-1,y-1,z,y-1,x-2,x; ideal I=fitting(M,0); // 0-th Fitting ideal of coker(M) qring Q=I; matrix M=fetch(R,M); isLocallyFree(M,1); // as R/I-module, coker(M) is locally free of rk 1 isLocallyFree(M,0); } /////////////////////////////////////////////////////////////////////////////// proc flatteningStrat (module M) "USAGE: flatteningStrat(M); M module RETURN: list of ideals. The list entries L[1],...,L[r] describe the flattening stratification of M'=coker(M): setting L[0]=0, L[r+1]=1, the flattening stratification is given by the open sets Spec(A/V(L[i-1])) \ V(L[i]), i=1,...,r+1 (A = basering). NOTE: for more information see the book 'A Singular Introduction to Commutative Algebra' (by Greuel/Pfister, Springer 2002). EXAMPLE: example flatteningStrat; shows an example " { list l; int v,w; ideal F; while(1) { F=interred(fitting(M,w)); if(F[1]==1){return(l);} if(size(F)!=0){v++;l[v]=F;} w++; } return(l); } example {"EXAMPLE:"; echo=2; ring A = 0,x(0..4),dp; // presentation matrix: matrix M[2][4] = x(0),x(1),x(2),x(3),x(1),x(2),x(3),x(4); list L = flatteningStrat(M); L; } /////////////////////////////////////////////////////////////////////////////// proc isFlat(module M) "USAGE: isFlat(M); M module RETURN: 1 if M'=coker(M) is flat;@* 0 if this is not the case. EXAMPLE: example isFlat; shows an example. " { if (size(ideal(M))==0) {return(1);} int w; ideal F=fitting(M,0); while(size(F)==0) { w++; F=fitting(M,w); } if (deg(std(F)[1])==0) {return(1);} return(0); } example {"EXAMPLE:"; echo=2; ring A = 0,(x,y),dp; matrix M[3][3] = x-1,y,x,x,x+1,y,x2,xy+x+1,x2+y; print(M); isFlat(M); // coker(M) is not flat over A=Q[x,y] qring B = std(x2+x-y); // the ring B=Q[x,y]/ matrix M = fetch(A,M); isFlat(M); // coker(M) is flat over B setring A; qring C = std(x2+x+y); // the ring C=Q[x,y]/ matrix M = fetch(A,M); isFlat(M); // coker(M) is not flat over C } /////////////////////////////////////////////////////////////////////////////// proc flatLocus(module M) "USAGE: flatLocus(M); M module RETURN: ideal I, s.th. complement of V(I) is flat locus of coker(M). NOTE: computation is based on Fitting ideals;@* output is not radical (in general) EXAMPLE: example flatLocus; shows an example " { if (size(ideal(M))==0) {return(ideal(1));} int v,w; ideal F=fitting(M,0); while(size(F)==0) { w++; F=fitting(M,w); } if(typeof(basering)=="qring") { for(v=w+1;v<=nrows(M);v++) { F=F+intersect(fitting(M,v),quotient(ideal(0),fitting(M,v-1))); } } return(interred(F)); } example {"EXAMPLE:"; echo=2; ring R=0,(x,y,z),dp; matrix M[2][3]=x,y,z,0,x3,z3; ideal I=flatLocus(M); // coker(M) is flat outside V(x,yz) I; // computed ideal not radical ideal J=radical(I); J; qring r=std(J); matrix M=fetch(r,M); flatLocus(M); // coker(M) is flat over Spec(Q[x,y,z]/) isFlat(M); // flatness test } /////////////////////////////////////////////////////////////////////////////// proc isReg(ideal I, module N) "USAGE: isReg(I,M); I ideal, M module RETURN: 1 if given (ordered) list of generators for I is coker(M)-sequence;@* 0 if this is not the case. EXAMPLE: example isReg; shows an example. " { int n=nrows(N); int i; while(i 9) { return(-1); } // result too large for integer execute("int a ="+s); return(a); } static proc basisNumber(int n,intvec v) { int p=size(v); if(p==1){return(v[1]);} int j=n-1; int b; while(j>=n-v[1]+1) { b=b+binom_int(j,p-1); j--; } intvec w=v-v[1]; w=w[2..size(w)]; b=b+basisNumber(n-v[1],w); return(b); } static proc basisElement(int n,int p,int N) { if(p==1){return(N);} int s,R; while(R20) { ERROR("// too many generators in input ideal"); } if (p>n) { module hom=0; return(hom); } int a = binom_int(n,p-1); // n over p-1 independent of char(basering) int b = binom_int(n,p); matrix N = matrix(M); module ker= freemodule(nrows(N)); if(p!=0) { module im= tensor(unitmat(a),N); module f = tensor(KoszulMap(x,p),unitmat(nrows(N))); ker = modulo(f,im); } module im1 = tensor(unitmat(b),N); module im2 = tensor(KoszulMap(x,p+1),unitmat(nrows(N))); module hom = modulo(ker,im1+im2); hom = prune(hom); return(hom); } example {"EXAMPLE:"; echo=2; ring R=0,x(1..3),dp; ideal x=maxideal(1); module M=0; KoszulHomology(x,M,0); // H_0(x,R), x=(x_1,x_2,x_3) KoszulHomology(x,M,1); // H_1(x,R), x=(x_1,x_2,x_3) qring S=std(x(1)*x(2)); module M=0; ideal x=maxideal(1); KoszulHomology(x,M,1); KoszulHomology(x,M,2); } /////////////////////////////////////////////////////////////////////////////// proc depth(module M,ideal #) "USAGE: depth(M,[I]); M module, I ideal RETURN: int, - if called with 1 argument: the depth of M'=coker(M) w.r.t. the maxideal in the basering (which is then assumed to be local)@* - if called with 2 arguments: the depth of M'=coker(M) w.r.t. the ideal I. NOTE: procedure makes use of KoszulHomology. EXAMPLE: example depth; shows an example. " { ideal m=maxideal(1); int n=size(m); int i; if (size(#)==0) { // depth(M') over local basering while(i,Q[x,y,z]) ring r=0,(x,y,z),ds; // local ring matrix M[2][2]=x,xy,1+yz,0; print(M); depth(M); // depth(maxideal,coker(M)) ideal I=x; depth(M,I); // depth(,coker(M)) I=x+z; depth(M,I); // depth(,coker(M)) } /////////////////////////////////////////////////////////////////////////////// proc isCM(module M) "USAGE: isCM(M); M module RETURN: 1 if M'=coker(M) is Cohen-Macaulay;@* 0 if this is not the case. ASSUME: basering is local. EXAMPLE: example isCM; shows an example " { // test if basering is local: ideal m=maxideal(1); int i; poly f=1; for (i=1; i<=size(m); i++) { f=f+m[i]; } if (ord(f)>0) { print("// basering must be local -- result has no meaning"); return(0); } return(depth(M)==dim(std(Ann(M)))); } example {"EXAMPLE:"; echo=2; ring R=0,(x,y,z),ds; // local ring R = Q[x,y,z]_ module M=xz,yz,z2; isCM(M); // test if R/ is Cohen-Macaulay M=x2+y2,z7; // test if R/ is Cohen-Macaulay isCM(M); }