p-adic exponential function

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In mathematics, particularly p-adic analysis, the p-adic exponential function is a p-adic analogue of the usual exponential function on the complex numbers. As in the complex case, it has an inverse function named the p-adic logarithm.

Definition[edit]

The usual exponential function on C is defined by the infinite series

\exp(z)=\sum_{n=0}^\infty \frac{z^n}{n!}.

Entirely analogously, one defines the exponential function on Cp, the completion of the algebraic closure of Qp, by

\exp_p(z)=\sum_{n=0}^\infty\frac{z^n}{n!}.

However, unlike exp which converges on all of C, expp only converges on the disc

|z|_p<p^{-1/(p-1)}.

This is because p-adic series converge if and only if the summands tend to zero, and since the n! in the denominator of each summand tends to make them very large p-adically, rather a small value of z is needed in the numerator.

p-adic logarithm function[edit]

The power series

\log(1+x)=\sum_{n=1}^\infty \frac{(-1)^{n+1}x^n}{n},

converges for x in Cp satisfying |x|p < 1 and so defines the p-adic logarithm function logp(z) for |z − 1|p < 1 satisfying the usual property logp(zw) = logpz + logpw. The function logp can be extended to all of C ×
p
 
(the set of nonzero elements of Cp) by imposing that it continues to satisfy this last property and setting logp(p) = 0. Specifically, every element w of C ×
p
 
can be written as w = pr·ζ·z with r a rational number, ζ a root of unity, and |z − 1|p < 1,[1] in which case logp(w) = logp(z).[2] This function on C ×
p
 
is sometimes called the Iwasawa logarithm to emphasize the choice of logp(p) = 0. In fact, there is an extension of the logarithm from |z − 1|p < 1 to all of C ×
p
 
for each choice of logp(p) in Cp.[3]

Properties[edit]

If z and w are both in the radius of convergence for expp, then their sum is too and we have the usual addition formula: expp(z + w) = expp(z)expp(w).

Similarly if z and w are nonzero elements of Cp then logp(zw) = logpz + logpw.

And for suitable z, so that everything is defined, we have expp(logp(z)) = z and logp(expp(z)) = z.

The roots of the Iwasawa logarithm logp(z) are exactly the elements of Cp of the form pr·ζ where r is a rational number and ζ is a root of unity.[4]

Note that there is no analogue in Cp of Euler's identity, e2πi = 1. This is a corollary of Strassmann's theorem.

Another major difference to the situation in C is that the domain of convergence of expp is much smaller than that of logp. A modified exponential function — the Artin–Hasse exponential — can be used instead which converges on |z|p < 1.

Notes[edit]

  1. ^ Cohen 2007, Proposition 4.4.44
  2. ^ In factoring w as above, there is a choice of a root involved in writing pr since r is rational; however, different choices differ only by multiplication by a root of unity, which gets absorbed into the factor ζ.
  3. ^ Cohen 2007, §4.4.11
  4. ^ Cohen 2007, Proposition 4.4.45

References[edit]

External links[edit]