A228138 - OEIS

A228138

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

4, 13, 20, 50, 63, 111, 130, 196, 221, 305, 336, 438, 475, 595, 638, 776, 825, 981, 1036, 1210, 1271, 1463, 1530, 1740, 1813, 2041, 2120, 2366, 2451, 2715, 2806, 3088, 3185, 3485, 3588, 3906, 4015, 4351, 4466, 4820, 4941, 5313, 5440, 5830, 5963, 6371, 6510

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OFFSET

1,1

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1).

FORMULA

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

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

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

PROG

(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))