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A122963
Triangular numbers with semiprime indices.
0
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.
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
KEYWORD
nonn
AUTHOR
Jonathan Vos Post, Oct 26 2006
EXTENSIONS
a(41)-a(46) from Giovanni Resta, Jun 13 2016
STATUS
approved