Artin–Hasse exponential

In mathematics, the Artin–Hasse exponential, introduced by Artin and Hasse (1928), is the power series given by

$E_{p}(x)=\exp \left(x+{\frac {x^{p}}{p}}+{\frac {x^{p^{2}}}{p^{2}}}+{\frac {x^{p^{3}}}{p^{3}}}+\cdots \right).$

Motivation

One motivation for considering this series to be analogous to the exponential function comes from infinite products. In the ring of formal power series Q[[x]] we have the identity

$e^{x}=\prod _{n\geq 1}(1-x^{n})^{-\mu (n)/n},$

where μ(n) is the Möbius function. This identity can be verified by showing the logarithmic derivative of the two sides are equal and that both sides have the same constant term. In a similar way, one can verify a product expansion for the Artin–Hasse exponential:

$E_{p}(x)=\prod _{(p,n)=1}(1-x^{n})^{-\mu (n)/n}.$

So passing from a product over all n to a product over only n prime to p, which is a typical operation in p-adic analysis, leads from e^x to E_p(x).

Properties

The coefficients of E_p(x) are rational. We can use either formula for E_p(x) to prove that, unlike e^x, all of its coefficients are p-integral; in other words, the denominators of the coefficients of E_p(x) are not divisible by p. A first proof uses the definition of E_p(x) and Dwork's lemma, which says that a power series f(x) = 1 + ... with rational coefficients has p-integral coefficients if and only if f(x^p)/f(x)^p ≡ 1 mod pZ_p[[x]]. When f(x) = E_p(x), we have f(x^p)/f(x)^p = e^−px, whose constant term is 1 and all higher coefficients are in pZ_p. A second proof comes from the infinite product for E_p(x): each exponent -μ(n)/n for n not divisible by p is a p-integral, and when a rational number a is p-integral all coefficients in the binomial expansion of (1 - xⁿ)^a are p-integral by p-adic continuity of the binomial coefficient polynomials t(t-1)...(t-k+1)/k! in t together with their obvious integrality when t is a nonnegative integer (a is a p-adic limit of nonnegative integers) . Thus each factor in the product of E_p(x) has p-integral coefficients, so E_p(x) itself has p-integral coefficients.

The (p-integral) series expansion has radius of convergence 1.

Combinatorial interpretation

The Artin–Hasse exponential is the generating function for the probability a uniformly randomly selected element of S_n (the symmetric group with n elements) has p-power order (the number of which is denoted by t_p,n):

$E_{p}(x)=\sum _{n\geq 0}{\frac {t_{p,n}}{n!}}x^{n}.$

This gives a third proof that the coefficients of E_p(x) are p-integral, using the theorem of Frobenius that in a finite group of order divisible by d the number of elements of order dividing d is also divisible by d. Apply this theorem to the nth symmetric group with d equal to the highest power of p dividing n!.

More generally, for any topologically finitely generated profinite group G there is an identity

$\exp(\sum _{H\subset G}x^{[G:H]}/[G:H])=\sum _{n\geq 0}{\frac {a_{G,n}}{n!}}x^{n},$

where H runs over open subgroups of G with finite index (there are finitely many of each index since G is topologically finitely generated) and a_G,n is the number of continuous homomorphisms from G to S_n. Two special cases are worth noting. (1) If G is the p-adic integers, it has exactly one open subgroup of each p-power index and a continuous homomorphism from G to S_n is essentially the same thing as choosing an element of p-power order in S_n, so we have recovered the above combinatorial interpretation of the Taylor coefficients in the Artin–Hasse exponential series. (2) If G is a finite group then the sum in the exponential is a finite sum running over all subgroups of G, and continuous homomorphisms from G to S_n are simply homomorphisms from G to S_n. The result in this case is due to Wohlfahrt (1977). The special case when G is a finite cyclic group is due to Chowla, Herstein, and Scott (1952), and takes the form

$\exp(\sum _{d|m}x^{d}/d)=\sum _{n\geq 0}{\frac {a_{m,n}}{n!}}x^{n},$

where a_m,n is the number of solutions to g^m = 1 in S_n.

David Roberts provided a natural combinatorial link between the Artin–Hasse exponential and the regular exponential in the spirit of the ergodic perspective (linking the p-adic and regular norms over the rationals) by showing that the Artin–Hasse exponential is also the generating function for the probability that an element of the symmetric group is unipotent in characteristic p, whereas the regular exponential is the probability that an element of the same group is unipotent in characteristic zero.

Conjectures

At the 2002 PROMYS program, Keith Conrad conjectured that the coefficients of $E_{p}(x)$ are uniformly distributed in the p-adic integers with respect to the normalized Haar measure, with supporting computational evidence. The problem is still open.

Dinesh Thakur has also posed the problem of whether the Artin–Hasse exponential reduced mod p is transcendental over $\mathbb {F} _{p}(x)$.

See also

• Witt vector
• Formal group

References

• Artin, E.; Hasse, H. (1928), "Die beiden Ergänzungssätze zum Reziprozitätsgesetz der ln-ten Potenzreste im Körper der ln-ten Einheitswurzeln", Abhandlungen Hamburg, 6: 146–162, JFM 54.0191.05
• A course in p-adic analysis, by Alain M. Robert
• Fesenko, Ivan B.; Vostokov, Sergei V. (2002), Local fields and their extensions, Translations of Mathematical Monographs, vol. 121 (Second ed.), Providence, RI: American Mathematical Society, ISBN 978-0-8218-3259-2, MR 1915966