Change of basis and Stirling numbers

Polynomials form a vector space-the sum of two polynomials is a polynomial etc.-and the most natural basis for this vector space is powers of x:

1, x, x^2, x^3, ...

But the power basis is not the only possible basis, and often not the most useful basis in application.

Falling powers

In some applications the falling powers of x are a more useful basis. For positive integers n, the n^th falling power of x is defined to be

Falling powers come up in combinatorics, in the calculus of finite differences, and in hypergeometric functions.

Change of basis

Since we have two bases for the vector space of polynomials, we can ask about the matrices that represent the change of basis from one to the other, and here's where we see an interesting connection.

The entries of these matrices are numbers that come up in other applications, namely the Stirling numbers. You can think of Stirling numbers as variations on binomial coefficients. More on Stirling numbers here.

In summation notation, we have

where the S₁ are the (signed) Stirling numbers of the 1st kind, and the S₂ are the Stirling numbers of the 2nd kind.

(There are two conventions for defining Stirling numbers of the 1st kind, differing by a factor of (-1)^n-k.)

Matrix form

This means the (i, j)th element of matrix representing the change of basis from the power basis to the falling power basis is S₁(i, j) and the (i, j)th entry of the matrix for the opposite change of basis is S₂(i, j). These are lower triangular matrices because S₁(i, j) and S₂(i, j) are zero for j > i.

These are infinite matrices since there's no limit to the degree of a polynomial. But if we limit our attention to polynomials of degree less than m, we take the upper left m by m submatrix of the infinite matrix. For example, if we look at polynomials of degree 4 or less, we have

to convert from powers to falling powers, and

going from falling powers to powers.

Incidentally, if we filled a matrix with unsigned Stirling numbers of the 1st kind, we would have the change of basis matrix going from the power basis to rising powers defined by

It may be hard to see, but there's a bar on top of the exponent n for rising powers whereas before we had a bar under the n for falling powers.

Related posts

• Stirling numbers
• Finite differences
• Inverting a power series