### Creating an array of variables

Here is a very very basic question.

I want to create a polynomial, say
a_0*x^0 + a_1*x + a_2*x^2+ \cdots + a_{20} x^{20}.

I could define these a_i one at a time, but it would be much better to have a way to create an array A of length 20 where A[i] is the coefficient a_i. The idea is that I want to do some operations and solve for these coefficients, which will end up being rational numbers.

There must be some very basic command that I don't know, but I can't find it in the documentation.