1,1

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.

Eric Weisstein's World of Mathematics, Hexagonal Number.

Eric Weisstein's World of Mathematics, Almost Prime.

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.

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.

Cf. A000384, A001222, A046308.

Sequence in context: A080854 A114821 A108098 this_sequence A045072 A100409 A043157

Adjacent sequences: A114502 A114503 A114504 this_sequence A114506 A114507 A114508

easy,nonn

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