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 A203625 Indices of octagonal numbers which are also decagonal. 2
 1, 135, 26125, 5068051, 983175705, 190731018655, 37000834443301, 7177971150981675, 1392489402456001585, 270135766105313325751, 52404946135028329194045, 10166289414429390550318915, 1972207741453166738432675401, 382598135552499917865388708815 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS As n increases, this sequence is approximately geometric with common ratio r = lim(n->Infinity, a(n)/a(n-1)) = (2+sqrt(3))^4 = 97+56*sqrt(3). LINKS Index entries for linear recurrences with constant coefficients, signature (195, -195, 1). FORMULA G.f.: x*(1-60*x-5*x^2) / ((1-x)*(1-194*x+x^2)). a(n) = 194*a(n-1)-a(n-2)-64. a(n) = 195*a(n-1)-195*a(n-2)+a(n-3). a(n) = 1/24*((1+2*sqrt(3))*(2+sqrt(3))^(4*n-3)+(1-2*sqrt(3))*(2-sqrt(3))^(4*n-3)+8). a(n) = ceiling(1/24*(1+2*sqrt(3))*(2+sqrt(3))^(4*n-3)). EXAMPLE The second octagonal number that is also decagonal is A000567(135) = 54405. Hence a(2) = 135. MATHEMATICA LinearRecurrence[{195, -195, 1}, {1, 135, 26125}, 14] CROSSREFS Cf. A203624, A203626, A001107, A000567. Sequence in context: A132054 A273440 A106175 * A051307 A056740 A065663 Adjacent sequences:  A203622 A203623 A203624 * A203626 A203627 A203628 KEYWORD nonn,easy AUTHOR Ant King, Jan 05 2012 STATUS approved

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Last modified September 27 03:21 EDT 2020. Contains 337380 sequences. (Running on oeis4.)