Series multisection

In mathematics, a multisection of a power series is a new power series composed of equally spaced terms extracted unaltered from the original series. Formally, if one is given a power series

\sum _{n=-\infty }^{\infty }a_{n}\cdot z^{n}

then its multisection is a power series of the form

\sum _{m=-\infty }^{\infty }a_{qm+p}\cdot z^{qm+p}

where p, q are integers, with 0 ≤ p < q. Series multisection represents one of the common transformations of generating functions.

Multisection of analytic functions

A multisection of the series of an analytic function

f(z)=\sum _{n=0}^{\infty }a_{n}\cdot z^{n}

has a closed-form expression in terms of the function $f(x)$ :

\sum _{m=0}^{\infty }a_{qm+p}\cdot z^{qm+p}={\frac {1}{q}}\cdot \sum _{k=0}^{q-1}\omega ^{-kp}\cdot f(\omega ^{k}\cdot z),

where $\omega =e^{\frac {2\pi i}{q}}$ is a primitive q-th root of unity. This expression is often called a root of unity filter. This solution was first discovered by Thomas Simpson.^[1] This expression is especially useful in that it can convert an infinite sum into a finite sum. It is used, for example, in a key step of a standard proof of Gauss's digamma theorem, which gives a closed-form solution to the digamma function evaluated at rational values p/q.

Examples

Bisection

In general, the bisections of a series are the even and odd parts of the series.

Geometric series

Consider the geometric series

\sum _{n=0}^{\infty }z^{n}={\frac {1}{1-z}}\quad {\text{ for }}|z|<1.

By setting $z\rightarrow z^{q}$ in the above series, its multisections are easily seen to be

\sum _{m=0}^{\infty }z^{qm+p}={\frac {z^{p}}{1-z^{q}}}\quad {\text{ for }}|z|<1.

Remembering that the sum of the multisections must equal the original series, we recover the familiar identity

\sum _{p=0}^{q-1}z^{p}={\frac {1-z^{q}}{1-z}}.

Exponential function

The exponential function

e^{z}=\sum _{n=0}^{\infty }{z^{n} \over n!}

by means of the above formula for analytic functions separates into

\sum _{m=0}^{\infty }{z^{qm+p} \over (qm+p)!}={\frac {1}{q}}\cdot \sum _{k=0}^{q-1}\omega ^{-kp}e^{\omega ^{k}z}.

The bisections are trivially the hyperbolic functions:

\sum _{m=0}^{\infty }{z^{2m} \over (2m)!}={\frac {1}{2}}\left(e^{z}+e^{-z}\right)=\cosh {z}

\sum _{m=0}^{\infty }{z^{2m+1} \over (2m+1)!}={\frac {1}{2}}\left(e^{z}-e^{-z}\right)=\sinh {z}.

Higher order multisections are found by noting that all such series must be real-valued along the real line. By taking the real part and using standard trigonometric identities, the formulas may be written in explicitly real form as

\sum _{m=0}^{\infty }{z^{qm+p} \over (qm+p)!}={\frac {1}{q}}\cdot \sum _{k=0}^{q-1}e^{z\cos(2\pi k/q)}\cos {\left(z\sin {\left({\frac {2\pi k}{q}}\right)}-{\frac {2\pi kp}{q}}\right)}.

These can be seen as solutions to the linear differential equation $f^{(q)}(z)=f(z)$ with boundary conditions $f^{(k)}(0)=\delta _{k,p}$ , using Kronecker delta notation. In particular, the trisections are