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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
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refs;
listen;
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OFFSET
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1,2
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COMMENTS
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Unit together with natural numbers n with number of nontrivial divisors equal to a perfect power. - Juri-Stepan Gerasimov, Oct 30 2009
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LINKS
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FORMULA
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EXAMPLE
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. 1
. 2 3
. 4 6 8
. 12 16 24 48 therefore 48 is a term: a(12) = 48;
. 1
. 2 3
. 4 6 8
. 9 12 16 18
. 24 36 48 72 144 therefore 48 is a term: a(29) = 144.
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PROG
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(Haskell)
a081619 n = a081619_list !! (n-1)
a081619_list = filter ((== 1) . a010054 . a000005) [1..]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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