| Singular https://www.singular.uni-kl.de/forum/ |
|
| Exact square root. https://www.singular.uni-kl.de/forum/viewtopic.php?f=10&t=2758 |
Page 1 of 1 |
| Author: | IliaToli [ Fri May 18, 2018 10:15 am ] |
| Post subject: | Exact square root. |
I have done this before but forgotten it now. How do you work in a ring of characteristic 0 where the square root of 2 is symbolic rather than a number? Thank you very much. Best regards. |
|
| Author: | gorzel [ Fri May 18, 2018 2:53 pm ] |
| Post subject: | Re: Exact square root. |
Work with an algebraic extension of the rationals numbers by definining a minimal polynomial. Code: > ring ra = (0,a),(x,y),dp;
> minpoly = a^2-2; > a^2; 2 > poly f = ax2-1; > f^2; 2*x4+(-2a)*x2+1 > basering; // coefficients: QQ[a]/(a2-2) // number of vars : 2 // block 1 : ordering dp // : names x y // block 2 : ordering C |
|
| Author: | IliaToli [ Sat Jun 09, 2018 11:56 pm ] |
| Post subject: | Re: Exact square root. |
Thank you very much, that worked like a charm. What if I want to involve other things, like Sqrt[3]? Or what if I want to work with square roots and parameters at the same time? The following won't work. > ring ra = (0, a, L), (x, y), dp; > minpoly = a^2 - 2; Best regards. |
|
| Author: | gorzel [ Mon Jun 11, 2018 4:29 pm ] |
| Post subject: | Re: Exact square root. |
At present, the syntax of Singular allows only to define algebraic extensions of numbers as Q, and Z_p. If you want to work over the field Q(a,b) with say a:=sqrt{2} and b:=sqrt{3} then you have to calculate first a primitive element c(this exists in characteristic 0), such that Q(c) = Q(a,b) and express a and b by c. In practice a generic lineare combination of the algebraic elements a and b is primitive element c. The library primitive.lib http://www.singular.uni-kl.de/Manual/latest/sing_1320.htm#SEC1396 does this task. (But there are two drawbacks: 1. IMO, the docmention is difficult to understand 2. the (random) delivered minimal polynomials have in general large coefficients which slows down a computation. The returned minimal polynomial should be further simplified e.g. by using a the LLL-algorithm.) To your example: The minimal polynomial for a+b is c4-10c2+1=0. You will find it by squaring a+b, using a2=2, b2=3, and squaring c2-5=2ab again. With Singular it can be done quickly as follows: Code: // find a primitive element > ring rabc = 0,(a,b,c),dp; > ideal I = a2-2,b2-3,c-a-b; > eliminate (I,ab); _[1]=c4-10c2+1 Check that this polynomial is irreducible over the rationals: Code: > ring rc =0,c,dp; > factorize(c4-10c2+1); [1]: _[1]=1 _[2]=c4-10c2+1 [2]: 1,1 Now find how to express a and b by c (it also shows what -a and -b is; you only need one of them): Code: > ring rcab = (0,c),(a,b),dp; minpoly = c4-10c2+1; > factorize (a2-2); [1]: _[1]=1 _[2]=a+(-1/2c3+9/2c) _[3]=a+(1/2c3-9/2c) [2]: 1,1,1 > factorize (b2-3); [1]: _[1]=1 _[2]=b+(1/2c3-11/2c) _[3]=b+(-1/2c3+11/2c) [2]: 1,1,1 Remark: The programm pari/gp https://pari.math.u-bordeaux.fr/ tells us that also c^4 - 4*c^2 + 1 =0 defines the same number field. As an alternative approach, which also addresses to your second question for rings with parameters and algebraic extensions simultaneously, you can mimic the (double) algebraic extension as follows. Instead of the parameter a and minpoly = ... define the variables a,b and take the defining relations a2-2,b2-3 into your ideal. Then compute a (reduced) Groebner basis. In this way you can also use additional parameters in the coefficent field: Code: > ring rt = (0,t),(x,y,a,b),dp; > // option(redSB); > ideal I = ....; > I = I,a2-2,b2-3; > ideal I = std(I); To display the result with a and b as parameters switch to another ring: Code: > ring rtab = (0,a,b,t),(x,y),dp;
> ideal Istd = imap(rt,Istd); > Istd; |
|
| Author: | IliaToli [ Thu Aug 02, 2018 5:30 am ] |
| Post subject: | Re: Exact square root. |
Now (sometime back) I figured out, and it is really easy. You can have as many as you want of these things, while staying in Q. Just add variables d1, d2, d3... in the end of all variables, and add the equations, for example, d1^2-2, d2^3-5, d3^2-7 ....................... Ilia Toli |
|
| Page 1 of 1 | All times are UTC + 1 hour [ DST ] |
| Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group http://www.phpbb.com/ |
|