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A114559 Numbers n such that n-th heptagonal number is 7-almost prime. 1
60, 63, 72, 114, 144, 159, 167, 180, 183, 207, 216, 225, 247, 255, 275, 297, 312, 315, 320, 330, 343, 352, 360, 378, 387, 391, 399, 405, 408, 411, 416, 420, 429, 440, 447, 448, 450, 459, 465, 468, 483, 486, 504, 513, 520, 525, 531, 546, 588, 591, 594, 609 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Hep(2) = 7 is the only prime heptagonal number.
LINKS
Eric Weisstein's World of Mathematics, Heptagonal Number.
Eric Weisstein's World of Mathematics, Almost Prime.
FORMULA
n such that Hep(n) = n*(5*n-3)/2 is 7-almost prime. n such that A000566(n) is an element of A046308. n such that A001222(A000566(n)) = 7. n such that A001222(n*(5*n-3)/2) = 7.
EXAMPLE
a(1) = 60 because Hep(60) = 60*(5*60-3)/2 = 8910 = 2 * 3^4 * 5 * 11 is 7-almost prime.
a(2) = 63 because Hep(63) = 63*(5*63-3)/2 = 9828 = 2^2 * 3^3 * 7 * 13 is 7-almost prime.
a(3) = 72 because Hep(72) = 72*(5*72-3)/2 = 12852 = 2^2 * 3^3 * 7 * 17 is 7-almost prime.
a(4) = 114 because Hep(114) = 114*(5*114-3)/2 = 32319 = 3^5 * 7 * 19 is 7-almost prime.
MATHEMATICA
Select[Range[250], PrimeOmega[(#(5#-3))/2]==7&] (* Harvey P. Dale, May 02 2019 *)
CROSSREFS
Sequence in context: A256749 A280183 A181463 * A175102 A344811 A295697
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Feb 15 2006
EXTENSIONS
More terms from Harvey P. Dale, May 02 2019
STATUS
approved

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Last modified March 28 13:42 EDT 2024. Contains 371254 sequences. (Running on oeis4.)