1,1

Triangular(p) = p*(p+1)/2 (see A034953).

Table of n, a(n) for n=1..5.

a(2) = 3 because 3 is the least prime such that the following are two primes:

p1 = 3 * 4 / 2 + 1 = 7.

p2 = 7 * 8 / 2 + 1 = 29.

a(3) = 43 because 43 is the least prime such that the following are three primes:

p1 = 43 * 44 / 2 + 1 = 947.

p2 = 947 * 948 / 2 + 1 = 448879.

p3 = 448879 * 448880 / 2 + 1 = 100746402761.

Cf. A000040, A000217, A034953, A231847, A231988.

Sequence in context: A308645 A086889 A269909 * A059495 A174325 A290366

Adjacent sequences: A231986 A231987 A231988 * A231990 A231991 A231992

nonn,hard,more

Alex Ratushnyak, Nov 16 2013

approved