The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A114505 Numbers n such that the n-th hexagonal number is a 7-almost prime. 1
 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; text; 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 Harvey P. Dale, Table of n, a(n) for n = 1..1000 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. MATHEMATICA Select[Range[800], PrimeOmega[#(2#-1)]==7&] (* Harvey P. Dale, Jul 20 2013 *) Position[PrimeOmega[PolygonalNumber[6, Range[700]]], 7]//Flatten (* Harvey P. Dale, Jan 10 2024 *) CROSSREFS Cf. A000384, A001222, A046308. Sequence in context: A335216 A114821 A108098 * A323610 A186400 A205188 Adjacent sequences: A114502 A114503 A114504 * A114506 A114507 A114508 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Feb 14 2006 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 12 09:33 EDT 2024. Contains 375850 sequences. (Running on oeis4.)