A105555 - OEIS

%I #14 May 25 2017 10:34:39

%S 2,3,3,5,3,7,3,7,5,7,3,13,3,7,7,11,3,13,3,13,7,7,3,19,5,7,7,13,3,19,3,

%T 13,7,7,7,23,3,7,7,19,3,19,3,13,13,7,3,29,5,13,7,13,3,19,7,19,7,7,3,

%U 37,3,7,13,17,7,19,3,13,7,19,3,37,3,7,13,13,7,19,3,29,11,7,3,37,7,7,7,19,3

%N Let d = number of divisors of n; a(n) = d-th prime.

%H Antti Karttunen, <a href="/A105555/b105555.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A000040(A000005(n)). - _Antti Karttunen_, May 25 2017

%e n = 6 has 4 divisors, prime(4) = 7, so a(6) = 7.

%t Prime[DivisorSigma[0,Range[90]]] (* _Harvey P. Dale_, Jul 27 2011 *)

%o (PARI) d(n) = for(x=1,n,print1(prime(numdiv(x))","))

%o (Python)

%o from sympy import prime, divisor_count

%o def a(n): return prime(divisor_count(n)) # _Indranil Ghosh_, May 25 2017

%Y Cf. A000005, A000040, A105560, A105561.

%K easy,nonn

%O 1,1

%A _Cino Hilliard_, May 03 2005