# Power series

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In mathematics, a power series is an infinite series whose terms involve successive powers of a variable, typically with real or complex coefficients. If the series converges, its value determines a function of the variable involved. Conversely, given a function it may be possible to form a power series from successive derivatives of the function: this Taylor series is then a power series in its own right.

Formally, let z be a variable and  be a sequence of real or complex coefficients. The associated power series is

.

## Contents

Over the complex numbers the series will have a radius of convergence R, a real number with the property that the series converges for all complex numbers z with  and that R is the "largest" number with this property (supremum of all numbers with this property. If the series converges for all complex numbers, we formally say that the radius of convergence is infinite.

For example

 converges only for  and has radius of convergence zero.
 converges for all , but diverges for  and so has radius of convergence 1.
 converges for all complex numbers z and so has radius of convergence infinity.

More generally we may consider power series in a complex variable  for a fixed complex number a.

Within the radius of convergence, a power series determines an analytic function of z. Derivatives of all orders exist, and the Taylor series exists and is equal to the original power series.

### Convergence tests

Some of the standard test for convergence of series translate into computations of the radius of convergence R.

• D'Alembert ratio test: if the limit of the sequence  exists, then this is equal to 1/R.
• Cauchy n-th root test: if the limit of the sequence  exists, then this is equal to 1/R.

## Algebra of power series

Power series may be added and multiplied. If  and  are power series, we may define their sum and product




and these purely algebraic definitions are consistent with the values achieved within the region of convergence.

If a power series g has constant term , then the n-th power of g involves only powers of z with exponent at least n. Hence if f denotes the series  it makes sense to consider the composite



as a power series in z, since any given power of z will appear in only finitely many of the terms . Again this purely algebraic definition is consistent with function composition within the region of convergence.

## Formal power series

Let R be any ring. A formal power series over R, with variable X is a formal sum  with coefficients . Addition and multiplication are now defined purely formally, with no questions of convergence, by the formulae above. The formal power series form another ring denoted .

## Inversion of power series

The power series  is called inverse series of the power series g, iff all elements of the expansion of  with respect to  are zero.

To simplify formulas, it is assumed that the zero-th element is zero, and the first coefficient is unity: , and . Then , and , and








and so on.