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A114505 Numbers n such that the n-th hexagonal number is a 7-almost prime. 0
48, 64, 68, 72, 80, 88, 96, 104, 108, 122, 140, 162, 168, 188, 203, 208, 216, 228, 230, 240, 243, 264, 272, 280, 308, 312, 324, 360, 378, 380, 396, 408, 410, 424, 428, 438, 440, 446, 450, 473, 486, 513, 518, 527, 544, 564, 567, 572, 578, 620, 638, 662, 666, 675, 689, 696 (list; graph; refs; listen; history; internal format)
OFFSET

1,1

COMMENTS

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 A046308. n such that A001222(A000384(n)) = 7. n such that A001222(n*(2*n-1)) = 7.

EXAMPLE

a(1) = 48 because HexagonalNumber(48) = H(48) = 48*(2*48-1) = 4560 = 2^4 * 3 * 5 * 19 is a 7-almost prime.

a(2) = 64 because H(64) = 64*(2*64-1) = 8128 = 2^6 * 127 is a 7-almost prime.

CROSSREFS

Cf. A000384, A001222, A046308.

Sequence in context: A080854 A114821 A108098 * A186400 A205188 A045072

Adjacent sequences:  A114502 A114503 A114504 * A114506 A114507 A114508

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post (jvospost3(AT)gmail.com), Feb 14 2006

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Last modified February 17 00:09 EST 2012. Contains 205978 sequences.