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A113619 Heptagon-free numbers. 1
1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 15, 16, 17, 19, 20, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32, 33, 37, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 69, 71, 73, 74, 75, 76, 78, 79, 80, 82, 83, 85, 86, 87, 88, 89, 92, 93, 94, 95, 96, 97, 99, 100 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Heptagonal number analogy of A112886 (the triangle-free positive integers).
LINKS
Eric Weisstein's World of Mathematics, Heptagonal Number.
FORMULA
{a(n)} = {integers k>1: no divisor of k is a heptagonal number n*(5*n-3)/2}>1}.
{a(n)} = {integers k>1: no divisor of k is in A000566 and >1}.
EXAMPLE
7 = Hep(2) is the first nontrivial heptagonal number, so neither 7 nor any multiple of 7 is in this sequence. 18 = Hep(3) is the first nontrivial heptagonal number, so neither 18 nor any multiple of 18 is in this sequence. 34 = Hep(4) is the first nontrivial heptagonal number, so neither 34 nor any multiple of 34 is in this sequence. Similarly for Hep(5) = 55, Hep(6) = 81, Hep(7) = 112. Hence three consecutive integers are excluded with 54 = 3*18, 55 = Hep(5), 56 = 7*8.
MATHEMATICA
upto=100; Module[{maxhep=Floor[(3+Sqrt[9+40upto])/10], heps}, heps= Rest[ Table[(n(5n-3))/2, {n, maxhep}]]; Complement[Range[upto], Union[ Flatten[ Table[n*heps, {n, Ceiling[upto/7]}]]]]] (* Harvey P. Dale, May 19 2012 *)
CROSSREFS
Sequence in context: A267305 A302478 A356939 * A325114 A004765 A247063
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Jan 14 2006
EXTENSIONS
Corrected by Harvey P. Dale, May 19 2012
STATUS
approved

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Last modified April 15 18:10 EDT 2024. Contains 371694 sequences. (Running on oeis4.)