OFFSET
0,2
COMMENTS
Lim_{n->infinity} a(n)/n^2 = Pi/4.
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
MATHEMATICA
a[n_] := (m = Ceiling[n/2]; Sum[ Boole[ n > 2*i && 2*i+n > 0 && n^2 - 4*i^2 + 4*j - 4*j^2 - 1 >= 0 ], {i, -m, m}, {j, -Ceiling @ Sqrt[ m^2-i^2 ], Ceiling @ Sqrt[ m^2-i^2 ]}]); Table[a[n], {n, 0, 50}] (* Jean-François Alcover, Jun 06 2013 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Jan 10 2000
STATUS
approved