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 A228138 Number of blocks in a Steiner system S(2, 4, A228137(n+1)). 2
 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 (list; graph; refs; listen; history; text; internal format)
 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)) CROSSREFS Cf. A228137. Sequence in context: A299801 A299677 A300309 * A339271 A081024 A339216 Adjacent sequences: A228135 A228136 A228137 * A228139 A228140 A228141 KEYWORD nonn,easy AUTHOR Colin Barker, Aug 12 2013 STATUS approved

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Last modified August 14 09:40 EDT 2024. Contains 375159 sequences. (Running on oeis4.)