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Coefficient of Polynomial Function in a loop

asked 2020-09-14 18:40:53 -0500

whatupmatt gravatar image

Hello so I am using the .coefficient function to extract the coefficient of a monomial given some polynomial. For example, 3x^(2).coefficient(x^(2)) is 3.

R.<x,y,z,w> = QQ[]
List1= [x^(2), y^(2),z^(2)]
List2= [x^(2)+y^(2)+z^(2), 3*x^(2),4*y^(2)]

For example if I do List2[0].coefficient(List1[0]), Sage immediately outputs 1. However, when I try to run it through all of List1, for some reason, the runtime seems to go forever so I feel I am doing something wrong. Even if I restrict to only the first term in List2, the runtime is very long and doesn't stop. Am I doing something wrong? In the code below, it should only run through 3 terms.

    while i<len(List1):
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answered 2020-09-14 23:23:30 -0500

updated 2020-09-14 23:24:46 -0500

The indentation in your second code block is wrong, but if you fix that (don't indent while, indent all of the lines after it), it works, at least for me. The code can also be rewritten a few ways:

No need for the variable i:

for a in List1: 
    Result = List2[0].coefficient(a) 

In fact, no need for an explicit loop at all, using list comprehension instead:

List3 = [List2[0].coefficient(a) for a in List1]

By the way, there are other ways to get at the coefficients. For example:

a = x^2 + 2*y^2 + 3*z^2
for b in a:

will print

(1, x^2)
(2, y^2)
(3, z^2)

You can get the same information using list comprehension, [b for b in a], or in fact just list(a).

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Asked: 2020-09-14 18:40:53 -0500

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Last updated: Sep 14