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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified December 1 08:20 EST 2022. Contains 358458 sequences. (Running on oeis4.)