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Number of blocks in a Steiner system S(2, 4, A228137(n+1)).
2

%I #12 Jun 13 2015 00:54:43

%S 4,13,20,50,63,111,130,196,221,305,336,438,475,595,638,776,825,981,

%T 1036,1210,1271,1463,1530,1740,1813,2041,2120,2366,2451,2715,2806,

%U 3088,3185,3485,3588,3906,4015,4351,4466,4820,4941,5313,5440,5830,5963,6371,6510

%N Number of blocks in a Steiner system S(2, 4, A228137(n+1)).

%H Colin Barker, <a href="/A228138/b228138.txt">Table of n, a(n) for n = 1..1000</a>

%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-2,-1,1).

%F a(n) = (1-(-1)^n+(-4+6*(-1)^n)*n+12*n^2)/4.

%F a(n) = a(n-1)+2*a(n-2)-2*a(n-3)-a(n-4)+a(n-5).

%F G.f.: x*(3*x^5-3*x^4-12*x^3+x^2-9*x-4) / ((x-1)^3*(x+1)^2).

%o (PARI) Vec(x*(3*x^5-3*x^4-12*x^3+x^2-9*x-4)/((x-1)^3*(x+1)^2) + O(x^99))

%Y Cf. A228137.

%K nonn,easy

%O 1,1

%A _Colin Barker_, Aug 12 2013

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