A122963 - OEIS

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

	OFFSET	`1,1`
	COMMENTS	`Semiprime analogue 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 = 45/2 = 10.` `a(2) = semiprime(2)(semiprime(2)+1)/2 = 67/2 = 21.` `a(3) = semiprime(3)(semiprime(3)+1)/2 = 910/2 = 45.`
	CROSSREFS	`Cf. A000217, A001358, A034953.` `Sequence in context: A086225 A014007 A074254 * A067520 A042309 A048697` `Adjacent sequences: A122960 A122961 A122962 * A122964 A122965 A122966`
	KEYWORD	`nonn`
	AUTHOR	`Jonathan Vos Post (jvospost3(AT)gmail.com), Oct 26 2006`

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Last modified February 16 04:47 EST 2012. Contains 205860 sequences.