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A169720
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a(n) = (3*2^(n-1)-1)*(3*2^n-1).
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8
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1, 10, 55, 253, 1081, 4465, 18145, 73153, 293761, 1177345, 4713985, 18865153, 75479041, 301953025, 1207885825, 4831690753, 19327057921, 77308821505, 309236465665, 1236948221953, 4947797606401, 19791199862785, 79164818325505, 316659311050753, 1266637319700481
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OFFSET
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0,2
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COMMENTS
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A subsequence of the triangular numbers A000217.
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LINKS
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FORMULA
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G.f.: (1 + 3*x - x^2)/((1-x)*(1-2*x)*(1-4*x)). - Paul D. Hanna, Apr 29 2010
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MATHEMATICA
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CoefficientList[Series[(1 + 3*x - x^2)/((1-x)*(1-2*x)*(1-4*x)), {x, 0, 30}], x] (* or *) LinearRecurrence[{7, -14, 8}, {1, 10, 55}, 30] (* Vincenzo Librandi, Dec 03 2012 *)
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PROG
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(PARI) a(n)=polcoeff((1+3*x-x^2)/((1-x)*(1-2*x)*(1-4*x)+x*O(x^n)), n) \\ Paul D. Hanna, Apr 29 2010
(Magma) I:=[1, 10, 55]; [n le 3 select I[n] else 7*Self(n-1)-14*Self(n-2)+8*Self(n-3): n in [1..30]]; // Vincenzo Librandi, Dec 03 2012
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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Alice V. Kleeva (alice27353(AT)gmail.com), Jan 19 2010
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STATUS
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approved
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