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Search: id:A077672
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| A077672 |
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a(1) = 1; for n > 1, a(n) = smallest triangular which is n times another triangular number > 1, or -1 if no such number exists. |
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+0
1
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| 1, 6, 45, -1, 15, 36, 21, 120, -1, 210, 66, 36, 78, 210, 45, -1
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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a(b^2)=-1 for b>1 because (2*b*m+b-1)^2<1+4*b^2*m^2+4*b^2*m<(2*b*m+b)^2 - Sascha Kurz (sascha.kurz(AT)uni-bayreuth.de), Jan 27 2003
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EXAMPLE
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a(5) = 15 = 5*3, a(6) = 630= 105*6.
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CROSSREFS
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Sequence in context: A124996 A001852 A065783 this_sequence A119202 A063947 A006086
Adjacent sequences: A077669 A077670 A077671 this_sequence A077673 A077674 A077675
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KEYWORD
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sign
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AUTHOR
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Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Nov 16 2002
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EXTENSIONS
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More terms from Sascha Kurz (sascha.kurz(AT)uni-bayreuth.de), Jan 27 2003
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