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A081619
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Numbers whose divisors can be arranged as equilateral triangle.
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2
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1, 4, 9, 12, 18, 20, 25, 28, 32, 44, 45, 48, 49, 50, 52, 63, 68, 75, 76, 80, 92, 98, 99, 112, 116, 117, 121, 124, 144, 147, 148, 153, 162, 164, 169, 171, 172, 175, 176, 188, 207, 208, 212, 236, 242, 243, 244, 245, 261, 268, 272, 275, 279, 284, 289, 292, 304, 316
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| A000005(a(n))=A000217(m) for some m; A000005(A081620(n))=n*(n+1)/2.
Unit together with natural numbers n with number of nontrivial divisors equal to a perfect power. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 30 2009]
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EXAMPLE
| Divisors of n=48 arranged as triangle:
.......... 1
........ 2...3
...... 4...6...8
... 12..16..24..48, therefore 48 is a term.
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CROSSREFS
| Cf. A000027, A001694, A081619, A144925. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 30 2009]
Sequence in context: A089910 A177880 A059269 * A038109 A067259 A060687
Adjacent sequences: A081616 A081617 A081618 * A081620 A081621 A081622
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KEYWORD
| nonn
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AUTHOR
| Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Mar 24 2003
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