Suppose we have given a left ideal L₀ = L. We are computing the descending sequence of
ideals L_k+1 = { a in L_k | ax_i in L_k for all i}, where x_i, i=1,..,n are the algebra variables.

The maximal two-sided ideal in L lies in the intersection of all L_k,
so we can get them as the invariants of the transition L_k ---> L_k+1.

We demonstrate the example for the algebra
U(sl₂) = < e, f, h | fe = ef - h, he = eh + 2e, hf = fh - 2f > and the ideal L = {e,h-a} , where a is considered as a parameter.

Computations