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A114455 Numbers n such that the n-th hexagonal number is a 4-almost prime. +0
1
12, 23, 25, 26, 27, 30, 33, 35, 39, 40, 42, 45, 46, 52, 53, 58, 59, 62, 65, 66, 70, 75, 76, 83, 85, 93, 94, 99, 111, 114, 117, 118, 119, 131, 133, 134, 137, 145, 146, 147, 154, 155, 161, 163, 167, 173, 174, 175, 178, 179, 183, 190, 193, 195 (list; graph; listen)
OFFSET

1,1

COMMENT

There are no prime hexagonal numbers. The n-th Hexagonal number A000384(n) = n*(2*n-1) is semiprime iff both n and 2*n-1 are prime iff A000384(n) is an element of A001358 iff n is an element of A005382.

LINKS

Eric Weisstein's World of Mathematics, Hexagonal Number.

Eric Weisstein's World of Mathematics, Almost Prime.

FORMULA

n such that hexagonal number A000384(n) is an element of A014613. n such that A001222(A000384(n)) = 4. n such that A001222(n*(2*n-1)) = 4.

EXAMPLE

a(1) = 12 because HexagonalNumber(12) = H(12) = 12*(2*12-1) = 276 = 2^2 * 3 * 23 is a 4-almost prime.

a(2) = 23 because H(23) = 23*(2*23-1) = 1035 = 3^2 * 5 * 23 is a 4-almost prime.

a(3) = 25 because H(25) = 25*(2*25-1) = 1225 = 5^2 * 7^2 is a 4-almost prime.

CROSSREFS

Cf. A000384, A001222, A014613.

Adjacent sequences: A114452 A114453 A114454 this_sequence A114456 A114457 A114458

Sequence in context: A014633 A066458 A101104 this_sequence A048992 A088783 A029756

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post (jvospost2(AT)yahoo.com), Feb 14 2006

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Last modified October 12 15:26 EDT 2008. Contains 144830 sequences.


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