A228142 - OEIS

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

%S 1,21,30,82,99,183,208,324,357,505,546,726,775,987,1044,1288,1353,

%T 1629,1702,2010,2091,2431,2520,2892,2989,3393,3498,3934,4047,4515,

%U 4636,5136,5265,5797,5934,6498,6643,7239,7392,8020,8181,8841,9010,9702,9879,10603

%N Number of blocks in a Steiner system S(2, 5, A228141(n+1)).

%H Colin Barker, <a href="/A228142/b228142.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) = (-3/4*(-1+(-1)^n)+(-5/2+3*(-1)^n)*n+5*n^2).

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

%F G.f.: -x*(12*x^3+7*x^2+20*x+1) / ((x-1)^3*(x+1)^2).

%o (PARI) Vec(-x*(12*x^3+7*x^2+20*x+1)/((x-1)^3*(x+1)^2) + O(x^100))

%Y Cf. A228141.

%K nonn

%O 1,2

%A _Colin Barker_, Aug 12 2013