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A123752
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a(n)=7a(n-2), a(0)=1, a(1)=2.
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0
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1, 2, 7, 14, 49, 98, 343, 686, 2401, 4802, 16807, 33614, 117649, 235298, 823543, 1647086, 5764801, 11529602, 40353607, 80707214, 282475249, 564950498, 1977326743, 3954653486, 13841287201, 27682574402, 96889010407, 193778020814
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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FORMULA
| a(n)=7^(n/2)*[7(1+(-1)^n)+2sqrt(7)*(1-(-1)^n)]/14.
O.g.f.: (1+2x)/(1-7x^2). a(2n)=A000420(n). a(2n+1)=2*A000420(n). - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jun 18 2008
a(n+3) = a(n+2)*a(n+1)/a(n). [Reinhard Zumkeller, Mar 04 2011]
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MAPLE
| a[0]:=1: a[1]:=2: for n from 2 to 30 do a[n]:=7*a[n-2] od: seq(a[n], n=0..30);
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MATHEMATICA
| f[0] = 1; f[1] = 2; f[n_] := f[n] = If[Mod[n, 2] == 1, 2*f[n - 1], (7/2)*f[n - 1]]; Table[f[n], {n, 0, 30}]
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CROSSREFS
| Cf. A018592.
Sequence in context: A128882 A018281 A018592 * A192507 A018622 A018668
Adjacent sequences: A123749 A123750 A123751 * A123753 A123754 A123755
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KEYWORD
| nonn
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AUTHOR
| Roger Bagula and Gary Adamson (rlbagulatftn(AT)yahoo.com), Nov 15 2006
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EXTENSIONS
| Edited by N. J. A. Sloane (njas(AT)research.att.com), Nov 29 2006
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