A078724 a(n) = prime(n*(n+1)/2+3).

5, 7, 13, 23, 41, 61, 89, 127, 167, 223, 271, 347, 419, 491, 593, 677, 797, 911, 1033, 1171, 1303, 1481, 1619, 1801, 1999, 2203, 2383, 2621, 2803, 3061, 3323, 3559, 3823, 4093, 4391, 4679, 4999, 5333, 5653, 5987, 6323, 6701, 7043, 7489, 7867, 8273, 8699, 9127

Offset: 0

Author

Cino Hilliard, Dec 20 2002

Keywords

• nonn
• easy

Comments

The sum of the reciprocals of the terms appears to converge.

Links

• Vincenzo Librandi, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A000040, A078721 (after 7, primes in third diagonal of triangle in comment).

Programs

Magma

[NthPrime(n*(n+1) div 2+3): n in [0..50]]; // Vincenzo Librandi, Jun 08 2016

Mathematica

Table[Prime[(n (n + 1)) / 2 + 3], {n, 0, 50}] (* Vincenzo Librandi, Jun 08 2016 *)

PARI

triprimes(n) = { sr = 0; for(j=1,n, x = j*(j+1)/2+2; z = prime(x); sr+=1.0/z; print1(z" "); ); print(); print(sr); }