From Encyclopedia of Mathematics - Reading time: 2 min
A formal Dirichlet series over a ring is associated to a function from the positive integers to
with addition and multiplication defined by
where
is the pointwise sum and
is the Dirichlet convolution of and .
The formal Dirichlet series form a ring , indeed an -algebra, with the zero function as additive zero element and the function defined by , for (so that ) as multiplicative identity. An element of this ring is invertible if is invertible in . If is commutative, so is ; if is an integral domain, so is . The non-zero multiplicative functions form a subgroup of the group of units of .
The ring of formal Dirichlet series over is isomorphic to a ring of formal power series in countably many variables.
The function is multiplicative if and only if there is a formal Euler identity beween the Dirichlet series and a formal Euler product over primes
and is totally multiplicative if the Euler product is of the form
References[edit]
- E.D. Cashwell, C.J. Everett,. "The ring of number-theoretic functions", Pacific J. Math. 9 (1959) 975-985 Zbl 0092.04602 MR0108510
- Henri Cohen, "Number Theory: Volume II: Analytic and Modern Tools", Graduate Texts in Mathematics 240, Springer (2008) ISBN 0-387-49894-X Zbl 1119.11002
- Gérald Tenenbaum, "Introduction to Analytic and Probabilistic Number Theory", Cambridge Studies in Advanced Mathematics 46, Cambridge University Press (1995) ISBN 0-521-41261-7 Zbl 0831.11001