Inverse function of A006450; primes in it; inverse function of primeth recurrence
Jonathan Post
jvospost3 at gmail.com
Mon Sep 3 21:54:49 CEST 2007
Am I right that OEIS does not yet have the inverse function of
A006450? Just as pi(n) is the inverse function of prime(n), so also
is pi(pi(n)) = A000720(A000720(n)) the inverse function of
prime(prime(n)) = A000040(A000040(n)).
pi(pi(n)) begins I think:
1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4,
4, 4, 4, 4, 4, 5
It is a step function, increasing by 1 precisely for those n which are
values of pi(n).
This also gives us the once-iterated analogue of A048989 pi(n) is
prime, namely the new seq pi(pi(n)) is prime:
5,6,7,8,9,10,11,12,13,14,15,16,31,32,33,34,35,36,37,38,39,40,59, 60, ...
We likewise a well-defined inverse function to the nice njas/rgwv
A007097 Primeth recurrence: a(n+1) = a(n)-th prime.
Are these worth submitting when vacation ends?
I considered this when Robert G. Wilson v and I worked on
A101184 a(n) = n + pi(n) + pi(pi(n)) + pi(pi(pi(n))) + pi(pi(pi(pi(n)))) + ...
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