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Search: id:A016970
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| 25, 121, 289, 529, 841, 1225, 1681, 2209, 2809, 3481, 4225, 5041, 5929, 6889, 7921, 9025, 10201, 11449, 12769, 14161, 15625, 17161, 18769, 20449, 22201, 24025, 25921, 27889, 29929, 32041, 34225
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OFFSET
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0,1
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COMMENT
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The product of 4 successive terms of an arithmetic progression + square of the common difference is a square: a(n) = the square arising as the sum of first four terms of an arithmetic progression + n^2 where 1 is the first term and n is the common difference. a(1) = 25 = 1*2*3*4+1 a(2) = 121 = 1*3*5*7 +2^2 a(3) = 289 = 1*4*7*10 + 3^2, etc. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Mar 25 2004
If Y is a fixed 2-subset of a (6n+1)-set X then a(n-1) is the number of 3-subsets of X intersecting Y. - Milan R. Janjic (agnus(AT)blic.net), Oct 21 2007
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LINKS
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Milan Janjic, Two Enumerative Functions
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CROSSREFS
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Sequence in context: A036057 A083509 A031151 this_sequence A062938 A141722 A090159
Adjacent sequences: A016967 A016968 A016969 this_sequence A016971 A016972 A016973
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KEYWORD
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nonn,easy
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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