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

1,1

Comments

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

Links

Table of n, a(n) for n=1..46.

Formula

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

Example

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.

Sequence in context: A240994 A014007 A074254 * A067520 A219884 A042309

Adjacent sequences: A122960 A122961 A122962 * A122964 A122965 A122966

Keyword

nonn

Author

Jonathan Vos Post, Oct 26 2006

Extensions

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

Status

approved