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A113508
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Pentagon-free numbers.
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3
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2, 3, 4, 6, 7, 8, 9, 11, 13, 14, 16, 17, 18, 19, 21, 23, 26, 27, 28, 29, 31, 32, 33, 34, 37, 38, 39, 41, 42, 43, 46, 47, 49, 52, 53, 54, 56, 57, 58, 59, 61, 62, 63, 64, 66, 67, 68, 69, 71, 73, 74, 76, 77, 79, 81, 82, 83, 86, 87, 89, 91, 93, 94, 97, 98, 99, 101
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Pentagonal number analogy of A112886 (the triangle-free positive integers).
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FORMULA
| {a(n)} = {integers k>1: no divisor of k is a pentagonal number n*(3*n-1)/2}>1}. {a(n)} = {integers k>1: no divisor of k is an element of A000326 > 1}.
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EXAMPLE
| 10 is not an element, since 10 = 2 * 5 and 5 is the first nontrivial pentagonal number. 24 is not an element, since 12|24 and 12 is a pentagonal number. 44 is not an element, since 22|44 and 22 is a pentagonal number.
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CROSSREFS
| Cf. A000326, A112886, A113502.
Sequence in context: A140467 A103677 A087919 * A059325 A004763 A030706
Adjacent sequences: A113505 A113506 A113507 * A113509 A113510 A113511
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KEYWORD
| easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Jan 11 2006
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