A116995 - OEIS

%I #6 Sep 16 2015 00:24:19

%S 5,12,35,70,176,247,425,532,782,1247,1426,2035,2501,2752,3290,4187,

%T 5192,5551,6700,7526,7957,9322,10292,11837,14065,15251,15862,17120,

%U 17767,19097,24130,25676,28085,28912,33227,34126,36895,39772,41750,44807

%N Pentagonal numbers with prime indices.

%C See also: A001248 Squares of primes. A034953 Triangular numbers (A000217) with prime indices.

%F a(n) = Prime(n)*(3*Prime(n)-1)/2. a(n) = A000326(A000040(n)).

%e a(1) = Pentagonal(prime(1)) = A000326(2) = 2*(3*2-1)/2 = 5.

%e a(2) = Pentagonal(prime(2)) = A000326(3) = 3*(3*3-1)/2 = 12.

%e a(3) = Pentagonal(prime(3)) = A000326(5) = 5*(3*5-1)/2 = 35.

%t Table[Prime[n]*(3*Prime[n] - 1)/2, {n, 1, 60}] (* _Stefan Steinerberger_, Apr 04 2006 *)

%Y Cf. A000040, A000326, A001248, A034953.

%K easy,nonn

%O 1,1

%A _Jonathan Vos Post_, Apr 02 2006