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{{DISPLAYTITLE:''p''-adic exponential function}} | |||
In [[mathematics]], particularly [[P-adic analysis|''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== | |||
The usual exponential function on '''C''' is defined by the infinite series | |||
:<math>\exp(z)=\sum_{n=0}^\infty \frac{z^n}{n!}.</math> | |||
Entirely analogously, one defines the exponential function on '''C'''<sub>''p''</sub>, the completion of the algebraic closure of '''Q'''<sub>''p''</sub>, by | |||
:<math>\exp_p(z)=\sum_{n=0}^\infty\frac{z^n}{n!}.</math> | |||
However, unlike exp which converges on all of '''C''', exp<sub>''p''</sub> only converges on the disc | |||
:<math>|z|_p<p^{-1/(p-1)}.</math> | |||
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== | |||
The power series | |||
:<math>\log(1+x)=\sum_{n=1}^\infty \frac{(-1)^{n+1}x^n}{n},</math> | |||
converges for ''x'' in '''C'''<sub>''p''</sub> satisfying |''x''|<sub>''p''</sub> < 1 and so defines the '''''p''-adic logarithm function''' log<sub>''p''</sub>(''z'') for |''z'' − 1|<sub>''p''</sub> < 1 satisfying the usual property log<sub>''p''</sub>(''zw'') = log<sub>''p''</sub>''z'' + log<sub>''p''</sub>''w''. The function log<sub>''p''</sub> can be extended to all of {{SubSup|'''C'''|''p''|×}} (the set of nonzero elements of '''C'''<sub>''p''</sub>) by imposing that it continue to satisfy this last property and setting log<sub>''p''</sub>(''p'') = 0. Specifically, every element ''w'' of {{SubSup|'''C'''|''p''|×}} can be written as ''w'' = ''p<sup>r''</sup>·ζ·''z'' with ''r'' a rational number, ζ a root of unity, and |''z'' − 1|<sub>''p''</sub> < 1,<ref>{{harvnb|Cohen|2007|loc=Proposition 4.4.44}}</ref> in which case log<sub>''p''</sub>(''w'') = log<sub>''p''</sub>(''z'').<ref>In factoring ''w'' as above, there is a choice of a root involved in writing ''p<sup>r</sup>'' since ''r'' is rational; however, different choices differ only by multiplication by a root of unity, which gets absorbed into the factor ζ.</ref> This function on {{SubSup|'''C'''|''p''|×}} is sometimes called the '''Iwasawa logarithm''' to emphasize the choice of log<sub>''p''</sub>(''p'') = 0. In fact, there is an extension of the logarithm from |''z'' − 1|<sub>''p''</sub> < 1 to all of {{SubSup|'''C'''|''p''|×}} for each choice of log<sub>''p''</sub>(''p'') in '''C'''<sub>''p''</sub>.<ref>{{harvnb|Cohen|2007|loc=§4.4.11}}</ref> | |||
==Properties== | |||
If ''z'' and ''w'' are both in the radius of convergence for exp<sub>''p''</sub>, then their sum is too and we have the usual addition formula: exp<sub>''p''</sub>(''z'' + ''w'') = exp<sub>''p''</sub>(''z'')exp<sub>''p''</sub>(''w''). | |||
Similarly if ''z'' and ''w'' are nonzero elements of '''C'''<sub>''p''</sub> then log<sub>''p''</sub>(''zw'') = log<sub>''p''</sub>''z'' + log<sub>''p''</sub>''w''. | |||
And for suitable ''z'', so that everything is defined, we have exp<sub>''p''</sub>(log<sub>''p''</sub>(''z'')) = ''z'' and log<sub>''p''</sub>(exp<sub>''p''</sub>(''z'')) = ''z''. | |||
The roots of the Iwasawa logarithm log<sub>''p''</sub>(''z'') are exactly the elements of '''C'''<sub>''p''</sub> of the form ''p<sup>r''</sup>·ζ where ''r'' is a rational number and ζ is a root of unity.<ref>{{harvnb|Cohen|2007|loc=Proposition 4.4.45}}</ref> | |||
Note that there is no analogue in '''C'''<sub>''p''</sub> of [[Euler's identity]], ''e''<sup>2''πi''</sup> = 1. This is a corollary of [[Strassmann's theorem]]. | |||
Another major difference to the situation in '''C''' is that the domain of convergence of exp<sub>''p''</sub> is much smaller than that of log<sub>''p''</sub>. A modified exponential function — the [[Artin–Hasse exponential]] — can be used instead which converges on |''z''|<sub>''p''</sub> < 1. | |||
==Notes== | |||
{{reflist}} | |||
==References== | |||
* Chapter 12 of {{cite book | last=Cassels | first=J. W. S. | authorlink=J. W. S. Cassels | title=Local fields | series=[[London Mathematical Society|London Mathematical Society Student Texts]] | publisher=[[Cambridge University Press]] | year=1986 | isbn=0-521-31525-5 }} | |||
*{{Citation | |||
| last=Cohen | |||
| first=Henri | |||
| author-link=Henri Cohen (number theorist) | |||
| title=Number theory, Volume I: Tools and Diophantine equations | |||
| publisher=Springer | |||
| location=New York | |||
| series=[[Graduate Texts in Mathematics]] | |||
| volume=239 | |||
| year=2007 | |||
| isbn=978-0-387-49922-2 | |||
| mr=2312337 | |||
| doi=10.1007/978-0-387-49923-9 | |||
}} | |||
==External links== | |||
* {{planetmath reference|id=7000|title=p-adic exponential and p-adic logarithm}} | |||
[[Category:Exponentials]] |
Revision as of 06:48, 24 December 2013
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
The usual exponential function on C is defined by the infinite series
Entirely analogously, one defines the exponential function on Cp, the completion of the algebraic closure of Qp, by
However, unlike exp which converges on all of C, expp only converges on the disc
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
The power series
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 Template:SubSup (the set of nonzero elements of Cp) by imposing that it continue to satisfy this last property and setting logp(p) = 0. Specifically, every element w of Template:SubSup 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 Template:SubSup 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 Template:SubSup for each choice of logp(p) in Cp.[3]
Properties
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
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References
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External links
- ↑ Template:Harvnb
- ↑ 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.
- ↑ Template:Harvnb
- ↑ Template:Harvnb