CODEWITHSUNDEEP
pythonsympysymbols
Brum
Brum

Reputation: 3

sympy.solve only solve in terms of certain symbols

I am trying to solve an equation using an ansatz with a number of independent terms that all have different coefficients. Sympy is trying to find an answer in terms of the independent terms instead of just the coefficients, I feel like I am missing something but have tried everything I could think of/google.

To show a minimal problem I have the following example:

from sympy import *
x,y,z= symbols('x y z')
a1, a2, a3 = symbols('a1 a2 a3')
expr = a1*x + a2*y + a3*z + y
solve(expr,[a1,a2,a3])

the result is:

a1 = -a3 z/x; a2 = -1

What I would want to have is:

 a1 = 0; a2 = -1; a3 = 0.

I feel like I am missing something, or is this plainly not possible with sympy.solve or another simple function. I could think of writing my own solver to solve the problem but if there is an 'easy' solution, that would be preferable.

Upvotes: 0

Views: 76

Answers (1)

smichr
smichr

Reputation: 19125

The coefficients of x,y,z must be zero in order for the sum to be zero (as you have stated) so that can be stated as:

from sympy import *
v=x,y,z= symbols('x y z')
a1, a2, a3 = symbols('a1 a2 a3')
expr = a1*x + a2*y + a3*z + y
expanded = expr.expand()
coeffs = [expanded.coeff(vi) for vi in v]  # you now have 3 equations
>>> solve(coeffs,[a1,a2,a3])
{a1: 0, a2: -1, a3: 0}

Upvotes: 1

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