Complex Factors of Real and Complex Polynomials – part 2

Last week I posted a Maxima utility to factor polynomials using complex roots.  The idea was to use the built-in solver solve to find each root a with multiplicity m, and then construct a factor of the form (z-a)^m.  Typically, solve can fail to identify some roots that are returned by the more robust to_poly_solve.  My problem at that time was that I didn’t know how to access the multiplicities for the roots returned by to_poly_solve.

I’ve written a utility to determine multiplicities for the roots returned by to_poly_solve. With that, here’s an updated version of factorC that takes advantage of the more robust solver.

/* factor a polynomial using all its complex roots */
factorC(_f,_z):=block(
[s,n,m,fp,j],
fp:1,
ss:to_poly_solve(_f,_z),
s:create_list(args(ss)[k][1],k,1,length(ss)),
m:tpsmult1(_f,_z,ss),
/*These lines were needed before I figured out how
 how to handle multiplicities with to_poly_solve
s:solve(_f,_z),
m:multiplicities,*/
n:length(s),
for j:1 thru n do 
 if lhs(s[j])#0 
 then fp:fp*(_z-(rhs(s[j])))^m[j],
 fp:fp*divide(_f,fp)[1],
fp
);
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