Find the coefficient of a certain term in an expression

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Rodolfo Batista Negri 2020-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/599401-find-the-coefficient-of-a-certain-term-in-an-expression

评论： Rodolfo Batista Negri 2020-9-25

Suppose I have:

syms X Y Z
A = 1.123 * X^3 + 4.234 * X * Y * Z + 5.32 * X^2 *Y

Is there a way to obtain the exact coefficient of, let's say:

X^2 * Y

from A automatically (usually I will have a much larger polynomial expression)?

In my application, I will have a large homogenous polynomial A. Eventually, I will choose a i, j and k, defining the trinomial:

and then I will have to obtain the corresponding coefficient

A_ijk

that is multiplying:

X^i * Y^j * Z^k

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Walter Roberson 2020-9-24

What is Ceofs ?

Rodolfo Batista Negri 2020-9-25

It is the array with all the coefficients taken from A

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回答（1 个）

Walter Roberson 2020-9-24

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Not directly, no.

However, you can coeffs(A,X, 'all') and look in the third last entry (highest degree is first so third last is degree 2). The result would be a summation of terms (for example it might be 5.32 * Y + 7 if the expression included 5.32*X^2*Y + 7*X^2 ) .

You can then coeffs() that entry for Y and pull out all but the constant term.