1,2

The number 2 appears only in column 1 of A217916 (when treated as a triangle).

Table of n, a(n) for n=1..37.

Involves triangular-indexed twin primes:

prime(a(n)*(a(n)-1)/2 + 1) - prime(a(n)*(a(n)-1)/2) = 2 for n>1 with a(1)=1.

Cf. A217916, A217918.

Sequence in context: A133097 A117176 A212987 * A127700 A101907 A242250

Adjacent sequences: A217916 A217917 A217918 * A217920 A217921 A217922

nonn

Paul D. Hanna, Mar 04 2013

approved