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Patrick Schulz
pschulz at posteo.de
Thu Sep 6 08:47:59 CEST 2018
Hi all!
I'm working with polynomials in one variable, where i want to compare
coefficients and remove some (simplification). For this, i need to
iterate over the individual coefficients, but in order to compare them,
i need them to be in a standard form. Since only terms with products and
sums are involved, i expand the expressions to remove any nested
expressions, e.g. R1 * (C1 + C2) -> R1 * C1 + R1 * C2.
This works well, but sometime terms of nth power also exist, which are
not expanded:
GiNaC::symbol R1("R1");
GiNaC::symbol R2("R2");
GiNaC::symbol C1("C1");
GiNaC::symbol C2("C2");
GiNaC::ex e = (R1 + R2) * (C1 + C2 * (R1 * C1));
std::cout << e << '\n' << e.expand() << '\n';
Here we get a term R1^2 * C1 * C2, which does not comply with my
"standard form".
In the end, ideally i would like to have only a sum of products, no
deeper nesting:
R1 * C1 + R1 * R1 * C1 * C2 + R2 * C1 + R1 * R2 * C1 * C2
Is there a way to do this?
Maybe some information what i'm trying to acchieve:
I solve spice netlists and extract transfer functions in s-space. For
the simplification (the removal of individual terms) i want to look at
the coefficients of the numerator and the denominator polynoms. This is
easy, since i use ex::coeff(). So now i have expressions for each power
of s. Then i want to store the individual elements in a vector, which
makes up the individual products. These are again stored in another
vector making up the sum of the polynoms. This works, kinda, but i have
to parse the ginac expression, which does not always behave
in a way i can foresee. Also i have a strong feeling that there is a
better way to do this, given that ginac has already many functions
dealing with polynoms. I'm happy to get some pointers on this topic :)
Thank you for your help!
Greetings,
Patrick
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