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Search: id:A048907
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| A048907 |
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Indices of 9-gonal numbers which are also triangular. |
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4
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| 1, 10, 154, 2449, 39025, 621946, 9912106, 157971745, 2517635809, 40124201194, 639469583290, 10191389131441, 162422756519761, 2588572715184730, 41254740686435914, 657487278267789889, 10478541711598202305
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Entries are == 1 (mod 3). - N. J. A. Sloane (njas(AT)research.att.com), Sep 22, 2007
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LINKS
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Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
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FORMULA
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G.f.: [1-7x+x^2]/[(1-x)(1-16x+x^2)].
a(n+2)=16*a(n+1)-a(n)-5, a(n+1)=8*a(n)-2.5+1.5*(28*a(n)^2-20*a(n)+1)^0.5 - Richard Choulet (richardchoulet(AT)yahoo.fr), Sep 22 2007
a(n)=(5/14)+(9/28)*{[8-3*sqrt(7)]^n+[8+3*sqrt(7)]^n}+(3/28)*sqrt(7)*{[8+3*sqrt(7)]^n- [8-3*sqrt(7)]^n}, with n>=0 [From Paolo P. Lava (ppl(AT)spl.at), Nov 25 2008]
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CROSSREFS
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Cf. A001080, A073352, A048908, A048909.
Cf. A048909 A048908.
Sequence in context: A116156 A025750 A034325 this_sequence A061654 A087603 A129460
Adjacent sequences: A048904 A048905 A048906 this_sequence A048908 A048909 A048910
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KEYWORD
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nonn
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AUTHOR
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Eric Weisstein (eric(AT)weisstein.com)
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