%I A018836
%S A018836 1,9,41,109,205,325,473,649,853,1085,1345,1633,1949,2293,2665,3065,3493,
%T A018836 3949,4433,4945,5485,6053,6649,7273,7925,8605,9313,10049,10813,11605,
%U A018836 12425,13273,14149,15053,15985,16945,17933,18949,19993,21065,22165
%N A018836 Number of squares on infinite chess-board at <= n knight's moves from 
               a fixed square..
%H A018836 Erich Friedman, <a href="a018836.gif">Illustration of initial terms</
               a>
%F A018836 G.f.: (1+5*x+12*x^2-8*x^4+4*x^5)*(1+x)/(1-x)^3;.
%F A018836 a(n)=1-6*n+14*n^2+4*Sign[n(n-1)(n-3)]. - Zak Seidov (zakseidov(AT)yahoo.com), 
               Mar 01 2005
%p A018836 (1+5*x+12*x^2-8*x^4+4*x^5)*(1+x)/(1-x)^3;
%t A018836 Table[1-6 n+14 n^2+4 Sign[n(n-1)(n-3)], {n, 0, 50}] (Seidov)
%Y A018836 Cf. A018842, A098498.
%Y A018836 Sequence in context: A000451 A000437 A095809 this_sequence A001846 A034441 
               A056243
%Y A018836 Adjacent sequences: A018833 A018834 A018835 this_sequence A018837 A018838 
               A018839
%K A018836 nonn,nice
%O A018836 0,2
%A A018836 N. J. A. Sloane (njas(AT)research.att.com), Marc LeBrun (mlb(AT)well.com)
%E A018836 More terms from Zak Seidov (zakseidov(AT)yahoo.com), Mar 01 2005