Given an arithmetical functionand a prime
we denote by
the power series
and call this the Bell series of
modulo
Bell series are very useful whenis multiplicative. We need the following theorem.
Theorem
The Bell series for an arithmetical function is unique. Supposeand
are multiplicative functions, then
if and only if
for all primes
Proof: Ifthen
for all
and all
so
Conversely if
for all
then
for all
Since
and
are multiplicative and take the same values for all prime powers they agree for all the positive integers so
Examples
Bell series for the Mobius function. Sinceand
for
we have
Bell series for Euler's totient functionSince
for
we have
Ifis a completely multiplicative function so that
for all
then the Bell series is a geometric series and
as long as
Hence we have the following