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A080756
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A positive integer n is in this sequence if it has infinitely many multiples that have exactly n positive divisors each.
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0
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8, 9, 12, 16, 18, 20, 24, 25, 27, 28, 32, 36, 40, 44, 45, 48, 49, 50, 52, 54, 56, 60, 63, 64, 68, 72, 75, 76, 80, 81, 84, 88, 90, 92, 96, 98, 99, 100, 104, 108, 112, 116, 117, 120, 121, 124, 125, 126, 128, 132, 135, 136, 140, 144, 147, 148, 150, 152, 153, 156, 160
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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COMMENTS
| Regional Math Competition for Northwestern Bulgaria, Vraca 2003, Problem 12/3
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FORMULA
| All non-squarefree positive integers except the number 4.
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EXAMPLE
| a(0)=8 because all numbers of the type 2^3p (p-odd prime) have exactly 8 divisors and are multiples of 8. Any squarefree number has only finite number of such multiples. The number 4 has only one such multiple (8).
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CROSSREFS
| Cf. A013929.
Sequence in context: A199635 A131864 A179443 * A189833 A063080 A167131
Adjacent sequences: A080753 A080754 A080755 * A080757 A080758 A080759
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KEYWORD
| nonn
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AUTHOR
| Ivaylo Kortezov (kortezov(AT)math.bas.bg), Mar 09 2003
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