A122963 Triangular numbers with semiprime indices.

10, 21, 45, 55, 105, 120, 231, 253, 325, 351, 561, 595, 630, 741, 780, 1081, 1225, 1326, 1540, 1653, 1711, 1953, 2145, 2415, 2775, 3003, 3403, 3655, 3741, 3828, 4186, 4371, 4465, 4560, 5671, 6216, 6670, 7021, 7140, 7381, 7503, 7626, 8385, 8911, 9045, 10011

Offset: 1

Author

Jonathan Vos Post, Oct 26 2006

Keywords

• nonn

Comments

Semiprime analog of A034953 Triangular numbers (A000217) with prime indices.

Examples

			a(1) = semiprime(1)*(semiprime(1)+1)/2 = 4*5/2 = 10.
a(2) = semiprime(2)*(semiprime(2)+1)/2 = 6*7/2 = 21.
a(3) = semiprime(3)*(semiprime(3)+1)/2 = 9*10/2 = 45.
		

Crossrefs

Cf. A000217, A001358, A034953.

Formula

a(n) = A000217(A001358(n)) = A001358(n)*(A001358(n)+1)/2.

Extensions

a(41)-a(46) from Giovanni Resta, Jun 13 2016