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

then its multisection is a power series of the form

where *p*, *q* are integers, with 0 ≤ *p* < *q*.

## Multisection of analytic functions

A multisection of the series of an analytic function

has a closed-form expression in terms of the function :

where is a primitive *q*-th root of unity. 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

By setting in the above series, its multisections are easily seen to be

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

### Exponential function

The exponential function

by means of the above formula for analytic functions separates into

The bisections are trivially the hyperbolic functions:

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

These can be seen as solutions to the linear differential equation with boundary conditions , using Kronecker delta notation. In particular, the trisections are

and the quadrisections are

### Binomial theorem

Multisection of a binomial expansion

at *x* = 1 gives the following identity for the sum of binomial coefficients with step *q*:

## References

**^**Simpson, Thomas (1757). "CIII. The invention of a general method for determining the sum of every 2d, 3d, 4th, or 5th, &c. term of a series, taken in order; the sum of the whole series being known".*Philosophical Transactions of the Royal Society of London*.**51**: 757–759. doi:10.1098/rstl.1757.0104.

- Weisstein, Eric W. "Series Multisection".
*MathWorld*. - Somos, Michael A Multisection of q-Series, 2006.
- John Riordan (1968).
*Combinatorial identities*. New York: John Wiley and Sons.