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Search: id:A133271
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| A133271 |
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Indices of 7-gonal numbers which are also centered 7-gonal numbers. |
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+0
1
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| 1, 8, 92, 1093, 13021, 155156, 1848848, 22031017, 262523353, 3128249216, 37276467236, 444189357613, 5292995824117, 63071760531788, 751568130557336, 8955745806156241, 106717381543317553, 1271652832713654392
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Numbers X such that 140*X^2-84*X-7 is a square.
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FORMULA
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a(n+2)=12*a(n+1)-a(n)-3 ; a(n+1)=6*a(n)-1.5+0.5*(140*a(n)^2-84*a(n)-7)^0.5
G.f.: -x*(1-5*x+x^2)/(-1+x)/(1-12*x+x^2). - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Nov 14 2007
a(n)=3/10+(7/20)*[6-sqrt(35)]^(n-1)+(1/20)*sqrt(35)*[6+sqrt(35)]^(n-1)+(7/20)*[6+sqrt(35)]^(n-1)-(1/20) *[6-sqrt(35)]^(n-1)*sqrt(35), with n>=1 - Paolo P. Lava (ppl(AT)spl.at), Jul 08 2008
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CROSSREFS
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Cf. A128919.
Sequence in context: A092956 A034667 A116149 this_sequence A099142 A007556 A027395
Adjacent sequences: A133268 A133269 A133270 this_sequence A133272 A133273 A133274
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KEYWORD
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nonn
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AUTHOR
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Richard Choulet (richardchoulet(AT)yahoo.fr), Oct 16 2007
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EXTENSIONS
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More terms from Paolo P. Lava (ppl(AT)spl.at), Jul 08 2008
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