A305900 - OEIS

%I #7 Jun 15 2018 09:26:30

%S 1,2,3,4,5,6,5,7,8,9,5,10,5,11,12,13,5,14,5,15,16,17,5,18,19,20,21,22,

%T 5,23,5,24,25,26,27,28,5,29,30,31,5,32,5,33,34,35,5,36,37,38,39,40,5,

%U 41,42,43,44,45,5,46,5,47,48,49,50,51,5,52,53,54,5,55,5,56,57,58,59,60,5,61,62,63,5,64,65,66,67,68,5,69,70,71,72,73,74,75,5,76

%N Filter sequence for a(primes > 3) = constant sequences.

%C For all i, j:

%C   a(i) = a(j) => A305801(i) = A305801(j) => A305800(i) = A305800(j).

%C   a(i) = a(j) => A007949(i) = A007949(j).

%C   a(i) = a(j) => A305893(i) = A305893(j).

%H Antti Karttunen, <a href="/A305900/b305900.txt">Table of n, a(n) for n = 1..100000</a>

%F For n <= 5, a(n) = n, for >= 5, a(n) = 5 when n is a prime, and a(n) = 3+n-A000720(n) when n is a composite.

%o (PARI) A305900(n) = if(n<=5,n,if(isprime(n),5,3+n-primepi(n)));

%Y Cf. A305800, A305801, A305893.

%Y Cf. also A305901, A305902, A305903 (this filter applied to various permutations of N).

%K nonn

%O 1,2

%A _Antti Karttunen_, Jun 14 2018