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A087125
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Indices n of hex numbers H(n) that are also triangular.
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2
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0, 5, 54, 539, 5340, 52865, 523314, 5180279, 51279480, 507614525, 5024865774, 49741043219, 492385566420, 4874114620985, 48248760643434, 477613491813359, 4727886157490160, 46801248083088245, 463284594673392294, 4586044698650834699, 45397162391834954700
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OFFSET
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0,2
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LINKS
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Colin Barker, Table of n, a(n) for n = 0..1000
Eric Weisstein's World of Mathematics, Hex Number
Index entries for linear recurrences with constant coefficients, signature (11,-11,1).
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FORMULA
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G.f.: (-x^2+5*x)/((1-x)*(1-10*x+x^2)).
a(n) = 11*a(n-1)-11*a(n-2)+a(n-3) for n>2. - Colin Barker, Jun 23 2015
a(n) = (-4-(5-2*sqrt(6))^n*(-2+sqrt(6))+(2+sqrt(6))*(5+2*sqrt(6))^n)/8. - Colin Barker, Mar 05 2016
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MATHEMATICA
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CoefficientList[Series[(-x^2+5*x)/((1-x)*(1-10*x+x^2)), {x, 0, 25}], x] (* G. C. Greubel, Nov 04 2017 *)
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PROG
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(PARI) concat(0, Vec(x*(x-5)/((x-1)*(x^2-10*x+1)) + O(x^50))) \\ Colin Barker, Jun 23 2015
(MAGMA) [Round((-4-(5-2*Sqrt(6))^n*(-2+Sqrt(6)) + (2+Sqrt(6))*(5 + 2*Sqrt(6))^n)/8): n in [0..25]]; // G. C. Greubel, Nov 04 2017
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CROSSREFS
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Cf. A006244, A031138.
Sequence in context: A221544 A268882 A229298 * A110430 A110432 A215542
Adjacent sequences: A087122 A087123 A087124 * A087126 A087127 A087128
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KEYWORD
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nonn,easy
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AUTHOR
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Eric W. Weisstein, Aug 14 2003
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STATUS
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approved
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