# Raising a power series by a power series?

I know addition and multiplication are well defined operations on formal power series. Now say you have two formal power series $F(x),G(x)\in R[[x]]$, with $R\supset\mathbb{Q}$ is the coefficient ring.

Is there a way to define $F(x)^{G(x)}$? Is there a standard well defined definition for this operation that hopefully satisfies the usual exponent laws? Thanks.

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No, even $x^x$ doesn't make sense in the power series ring ! – Georges Elencwajg Feb 14 '12 at 21:55

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At least, $F(x)$ probably needs to have a nonzero constant term, since otherwise you expect something like a logarithmic singularity (think of the expression as $\exp(G(x)\ln F(x))$). But then, I would suspect (but don't know) that this standard formula will provide a positive answer. – Harald Hanche-Olsen Feb 14 '12 at 22:00

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It interesting to note that $${\left( {1 + x + \frac{{{x^2}}}{{2!}} + \frac{{{x^3}}}{{3!}} + \cdots } \right)^x} = 1 + x^2 + \frac{{{{\left( {{x^2}} \right)}^2}}}{{2!}} + \frac{{{{\left( {{x^2}} \right)}^3}}}{{3!}} + \cdots$$ i.e $(e^x)^x = e^{x^2}$ – Peter Tamaroff Feb 18 '12 at 23:30

## 2 Answers

Composition of formal power series can be done via the power-series version of Faà di Bruno's formula. In that way one can find the series for $\log_e F(x)$. Then one can say $$F(x)^{G(x)} = e^{G(x)\log_e F(x)}.$$ Multiplication of formal power series is a well-known operation. Exponentiation can be done via the exponential formula.

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Provided $F(x)$ has a nonzero constant term. – Did Feb 14 '12 at 23:58

There is no general , universally accepted answer to this problem .If the exponent is a function of some variable , you cannot be restricted to the Real Number World . You have to get into the Complex Number World. f(x)=f(x)*1 =f(x)*exp(i*2*n*pi) f(x)^g(x) = [f(x)^g(x)] *[exp(i*2*n*pi*g(x))]

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What does this have to do with power series? – Martin Argerami Nov 28 '12 at 4:51
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