%N Consider the square lattice L and the sublattice K of index 5 spanned by (2,-1), (1,2); a(n) = number of points (x,y) in M with x >= 0, y >= 0, x+y <= n.
%H <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,0,0,1,-2,1)
%F G.f.: (x^3-x+1)/[(1-x^5)(1-x)^2].
%p a := proc(n) local i,t0,t1,t2; t0 := 0; for i from 0 to floor(2*n/5) do t1 := min(n-3*i,2*i); t2 := ceil(-i/2); t0 := t0+t1-t2+1; od; t0; end;
%A _N. J. A. Sloane_, Jul 20 2003