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 A137661 a(n+1) = if a(n) is prime then (number of primes so far) else (number of nonprimes so far), a(1) = 1. 2

%I

%S 1,1,2,1,3,2,3,4,4,5,5,6,6,7,7,8,8,9,10,11,9,12,13,10,14,15,16,17,11,

%T 12,18,19,13,14,20,21,22,23,15,24,25,26,27,28,29,16,30,31,17,18,32,33,

%U 34,35,36,37,19,20,38,39,40,41,21,42,43,22,44,45,46,47,23,24,48,49,50,51

%N a(n+1) = if a(n) is prime then (number of primes so far) else (number of nonprimes so far), a(1) = 1.

%C All terms greater than 1 occur exactly twice: a(A137662(n)) = a(n).

%H R. Zumkeller, <a href="/A137661/b137661.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n+1) = A005171(a(n))*Sum(A005171(a(k)):1<=k<=n) + A010051(a(n))*Sum(A010051(a(k)):1<=k<=n).

%K nonn

%O 1,3

%A _Reinhard Zumkeller_, Feb 04 2008

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Last modified December 3 12:33 EST 2020. Contains 338904 sequences. (Running on oeis4.)