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 A113920 G.f.: (x^3 - x + 1)^3/(x^3*(1 - x)^3). 0

%I

%S 1,0,0,3,3,3,6,9,12,16,21,27,34,42,51,61,72,84,97,111,126,142,159,177,

%T 196,216,237,259,282,306,331,357,384,412,441,471,502,534,567,601,636,

%U 672,709,747,786,826,867,909,952,996,1041,1087,1134,1182,1231,1281,1332,1384,1437

%N G.f.: (x^3 - x + 1)^3/(x^3*(1 - x)^3).

%C Series expansion of elliptical invariant for a cubic anharmonic group. Suggested by the anharmonic group elliptical invariant A078907.

%C If Y is a 4-subset of an n-set X then, for n>=7, a(n-3) is the number of 2-subsets of X which have no exactly one element in common with Y. - _Milan Janjic_, Dec 28 2007

%D McKean and Moll, Elliptic Curves, 1997, Cambridge University Press, page 20.

%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).

%F a(n-7)=1/2*n^2-9/2*n+16, n=7,8,9,... - _Milan Janjic_, Dec 28 2007

%t b = ReplacePart[Table[Coefficient[Series[(x^3 - x + 1)^3/(x^3*(1 - x)^3), {x, 0, 30}], x^n], {n, -3, 30}], 3, 4]

%Y Cf. A078907.

%K nonn,easy

%O -3,4

%A _Roger L. Bagula_, Jan 29 2006

%E Edited by _N. J. A. Sloane_, Apr 21 2007

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Last modified September 21 09:42 EDT 2020. Contains 337268 sequences. (Running on oeis4.)