%I #2 Mar 30 2012 18:59:44
%S 7,75,385,1365,3850,9282,19950,39270,72105,125125,207207,329875,
%T 507780,759220,1106700,1577532,2204475,3026415,4089085,5445825,
%U 7158382,9297750,11945050,15192450,19144125,23917257
%N The z^2 coefficients of the polynomials in the GF1 denominators of A156921.
%C See A157702 for background information.
%F a(n) = 7*a(n-1)-21*a(n-2)+35*a(n-3)-35*a(n-4)+21*a(n-5)-7*a(n-6)+a(n-7)
%F a(n) = 1/18*n^6+1/6*n^5+1/72*n^4-1/4*n^3-5/72*n^2+1/12*n
%F G.f.: (7+26*z+7*z^2)/(1-z)^7
%p nmax:=27; for n from 0 to nmax do fz(n):= product( (1-(2*m-1)*z)^(n+1-m) , m=1..n); c(n):= coeff(fz(n),z,2); end do: a:=n-> c(n): seq(a(n), n=2..nmax);
%Y Cf. A156921, A157702.
%K easy,nonn
%O 2,1
%A _Johannes W. Meijer_, Mar 07 2009