# How to find coordinates of an abstract vector relative to a basis

Let's say I have a degree 3 homogeneous polynomial over variables $x0, x1, x2.$ There are then 10 distinct monic monomials, forming a basis of the vector space of polynomials of degree 3. If I have another polynomial of degree 3 in $x0, x1, x2$, how can I find out the coordinates of the expansion, returning a list according to some chosen ordering of the monomials? So the desired output would be a list of 10 numbers. For example, let's say I want the ordering $x0^3, x0^2x1, x0^2x2, x0x1^2, x0x1x2, x0x2^2, x1^3, x1^2x2, x1x2^2, x2^3$ and I want the output corresponding to the polynomial $(x0+x1)^2x0$. Then I want the output to be the array {1, 2, 0, 1, 0, 0, 0, 0, 0, 0}.

The degree 3 was an arbitrary choice, and therefore as was the 10-dimensionality. So the method needs to at least work for fixed degree at a time, if not necessarily so that I can put in a degree at the very beginning for it to always work. Also, eventually I will want to take an array of polynomials and want to generate a list of lists of coefficients, so hopefully the method suggested will be amenable to this.

Thanks.

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Sounds like you want PolynomialReduce
base = {x^3, x^2 y, x^2 z, x y^2, xyz, x z^2, y^3, y^2 z, y z^2, z^3}

You might also find GroebnerBasis interesting, scholarpedia has a nice overview.