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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

Table of n, a(n) for n=1..14.

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 April 22 10:58 EDT 2019. Contains 322330 sequences. (Running on oeis4.)