# Complex polynomial variable transformation

I have two real polynomials that depend in variables (xi1, xi2) and parameters (eps, e2). I apply the following complexification:

w(xi1, xi2, eps, e2) = gamma1(xi1, xi2, eps, e2) + I gamma2(xi1, xi2, eps, e2)

Now I wish to define the variable z = xi1 + I xi2, and find a neat expression for w in terms of z and it’s conjugate, zc, as follows:

w(z, zc) = c30 z^3 + c21 z^2 zc+ c12 z zc^2 + c03 zc^3 + o(3)

In order to find the coefficients.

How may I ask this to Mathematica?

—————————- actual code ———————–

gamma1[xi1,xi2,eps,e2]=(0. – 1.72945 I) e2 (-0.3567 xi1 + 0.229 xi2 + 0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 – (0. + 0.5 I) (eps (0.5929 xi1 – 0.3806 xi2 – 1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) + 38.4563 e2 (-0.3567 xi1 + 0.229 xi2 + 0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);
gamma2[xi1,xi2,eps,e2]=(-1.72945 + 0. I) e2 (-0.3567 xi1 + 0.229 xi2 + 0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 + (0.5 + 0. I) (eps (0.5929 xi1 – 0.3806 xi2 – 1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) + 38.4563 e2 (-0.3567 xi1 + 0.229 xi2 + 0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);

w[xi1, xi2, eps, e2] = gamma1[xi1, xi2, eps, e2] + I gamma2 [xi1, xi2, eps, e2];

=================

Clarifying the question, providing the polynomials, and writing the mathematics in Mathematica format will elicit more help from readers, I believe.
– bbgodfrey
Dec 29 ’15 at 14:24

Hi! The polynomials I’m working on are very complicated, since they are obtained with several functions. I haven’t posted them to not make the question look too complicated. Anyhow, I’ll post the equations at the end of my question. Thank you for your advice.
– Mirko Aveta
Dec 29 ’15 at 14:46

I’ve written the polynomials explicitly, throwing away all the useless parts of the code.
– Mirko Aveta
Dec 29 ’15 at 15:01

=================

2

=================

gamma1[xi1, xi2, eps,
e2] = (0. –
1.72945 I) e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 – (0. +
0.5 I) (eps (0.5929 xi1 – 0.3806 xi2 –
1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) +
38.4563 e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);
gamma2[xi1, xi2, eps,
e2] = (-1.72945 +
0. I) e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 + (0.5 +
0. I) (eps (0.5929 xi1 – 0.3806 xi2 –
1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) +
38.4563 e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);
w[xi1, xi2, eps, e2] =
gamma1[xi1, xi2, eps, e2] + I gamma2[xi1, xi2, eps, e2];

Now form a Groebner basis from the reducing relations, ordered in such a way that the x variables are “larger” than the z variables. Use that to reduce the expression of interest.

gb = GroebnerBasis[{xi1+I*xi2-z, xi1-I*xi2-zc}, {xi1,xi2,zc,z}];
Expand[PolynomialReduce[w[xi1,xi2,eps,e2], gb, {xi1,xi2,zc,z}][[2]]]

(* Out[546]= (-0.0326007293462 –
0.00464029976304 I) e2 z^3 – (0.0369688588939 +
0.0632291173231 I) e2 eps z^3 + (0.0197005770425 –
0.0506048635599 I) e2 eps^2 z^3 + (0.012824498915 –
0.00395607930627 I) e2 eps^3 z^3 – (0.0533696753722 –
0.0831308436911 I) e2 z^2 zc – (0.122052644558 –
0.0838872079541 I) e2 eps z^2 zc – (0.0964865190488 –
0.0524172122716 I) e2 eps^2 z^2 zc – (0.0400623552714 –
0.00400907884547 I) e2 eps^3 z^2 zc + (0.0533696753722 +
0.0831308436911 I) e2 z zc^2 + (0.122052644558 +
0.0838872079541 I) e2 eps z zc^2 + (0.0964865190488 +
0.0524172122716 I) e2 eps^2 z zc^2 + (0.0400623552714 +
0.00400907884547 I) e2 eps^3 z zc^2 + (0.0326007293462 –
0.00464029976304 I) e2 zc^3 + (0.0369688588939 –
0.0632291173231 I) e2 eps zc^3 – (0.0197005770425 +
0.0506048635599 I) e2 eps^2 zc^3 – (0.012824498915 +
0.00395607930627 I) e2 eps^3 zc^3 *)

Smart! I think I need to take a better look at this GroebnerBasis function!
– Mirko Aveta
Dec 29 ’15 at 19:23

You have,

gamma1[xi1_, xi2_] = (0. – 1.72945 I) e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 – (0. +
0.5 I) (eps (0.5929 xi1 – 0.3806 xi2 –
1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) +
38.4563 e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);
gamma2[xi1_, xi2_] = (-1.72945 + 0. I) e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3 + (0.5 +
0. I) (eps (0.5929 xi1 – 0.3806 xi2 –
1.2296 (-0.0423 eps xi1 + 0.4227 eps xi2)) +
38.4563 e2 (-0.3567 xi1 + 0.229 xi2 +
0.7398 (-0.0423 eps xi1 + 0.4227 eps xi2))^3);

Just rewrite your variables in terms of z and zc,

Solve[{z == xi1 + I xi2, zc == xi1 – I xi2}, {xi1, xi2}]
(* {{xi1 -> (z + zc)/2, xi2 -> -(1/2) I (z – zc)}} *)

Then substitute them into the original expression:

gamma1[(z + zc)/2, -(1/2) I (z – zc)] // FullSimplify
gamma2[(z + zc)/2, -(1/2) I (z – zc)] // FullSimplify
(* (0.00641225 – 0.00197804 I) e2 ((0.838054 – 1.0568 I) z +
1. eps z – (0.612024 + 1.20191 I) zc – (0.98017 +
0.198158 I) eps zc)^3 – (0. +
0.5 I) (eps (((0.29645 +
0.1903 I) + (0.026006 + 0.259876 I) eps) z + ((0.29645 –
0.1903 I) + (0.026006 –
0.259876 I) eps) zc) + (0.043984 +
0.142584 I) e2 ((0.838054 – 1.0568 I) z +
1. eps z – (0.612024 + 1.20191 I) zc – (0.98017 +
0.198158 I) eps zc)^3) *)
(* (-0.00197804 – 0.00641225 I) e2 ((0.838054 – 1.0568 I) z +
1. eps z – (0.612024 + 1.20191 I) zc – (0.98017 +
0.198158 I) eps zc)^3 + (0.5 +
0. I) (eps (((0.29645 +
0.1903 I) + (0.026006 + 0.259876 I) eps) z + ((0.29645 –
0.1903 I) + (0.026006 –
0.259876 I) eps) zc) + (0.043984 +
0.142584 I) e2 ((0.838054 – 1.0568 I) z +
1. eps z – (0.612024 + 1.20191 I) zc – (0.98017 +
0.198158 I) eps zc)^3) *)

That’s the basic strategy, but you want a Taylor series in z and zc for w, so let’s try this:

Normal[
Series[gamma1[(z + zc)/2, -(1/2) I (z – zc)] +
I gamma2[(z + zc)/2, -(1/2) I (z – zc)], {z, 0, 3}, {zc, 0, 3}]]
(* ((-0.0326007 – 0.0046403 I) e2 – (0.0369689 +
0.0632291 I) e2 eps + (0.0197006 –
0.0506049 I) e2 eps^2 + (0.0128245 –
0.00395608 I) e2 eps^3) z^3 + ((-0.0533697 +
0.0831308 I) e2 – (0.122053 –
0.0838872 I) e2 eps – (0.0964865 –
0.0524172 I) e2 eps^2 – (0.0400624 –
0.00400908 I) e2 eps^3) z^2 zc + ((0.0533697 +
0.0831308 I) e2 + (0.122053 +
0.0838872 I) e2 eps + (0.0964865 +
0.0524172 I) e2 eps^2 + (0.0400624 +
0.00400908 I) e2 eps^3) z zc^2 + ((0.0326007 –
0.0046403 I) e2 + (0.0369689 –
0.0632291 I) e2 eps – (0.0197006 +
0.0506049 I) e2 eps^2 – (0.0128245 +
0.00395608 I) e2 eps^3) zc^3 *)

But this isn’t exactly what you want, it has terms like z^2 zc^2, so if you want it to match your expression exactly, you should use the SeriesCoefficient function.

wz = gamma1[(z + zc)/2, -(1/2) I (z – zc)] +
I gamma2[(z + zc)/2, -(1/2) I (z – zc)];
Total[SeriesCoefficient[
wz, {z, 0, #1}, {zc, 0, #2}] z^#1 zc^#2 & @@@ {{3, 0}, {2,
1}, {1, 2}, {0, 3}}]
(* (0.0128245 –
0.00395608 I) ((-2.21927 – 1.04643 I) e2 – (1.24345 +
5.31391 I) e2 eps + (2.51416 – 3.17039 I) e2 eps^2 + (1. +
0. I) e2 eps^3) z^3 + ((-0.0533697 +
0.0831308 I) e2 – (0.122053 –
0.0838872 I) e2 eps – (0.0964865 –
0.0524172 I) e2 eps^2 – (0.0400624 –
0.00400908 I) e2 eps^3) z^2 zc + ((0.0533697 +
0.0831308 I) e2 + (0.122053 +
0.0838872 I) e2 eps + (0.0964865 +
0.0524172 I) e2 eps^2 + (0.0400624 +
0.00400908 I) e2 eps^3) z zc^2 + ((0.0326007 –
0.0046403 I) e2 + (0.0369689 –
0.0632291 I) e2 eps – (0.0197006 +
0.0506049 I) e2 eps^2 – (0.0128245 +
0.00395608 I) e2 eps^3) zc^3 *)

Thank you very much!
– Mirko Aveta
Dec 29 ’15 at 15:18