OFFSET
0,3
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..500
EXAMPLE
a(2)=4 because a disk of diameter 2 covers the center of the circle and the 3 net points at distance 1.
MATHEMATICA
a[n_] := Sum[Boole[4*(i^2 + i*j + j^2) <= n^2 && Mod[i - j , 3] != 1], {i, -n, n}, {j, -n, n}];
Table[a[n], {n, 0, 52}] (* Jean-François Alcover, Oct 08 2017, translated from PARI *)
PROG
(PARI)
a(n) = sum(i=-n, n, sum(j=-n, n, 4*(i^2 + i*j + j^2) <= n^2 && (i-j) % 3 != 1)); \\ Andrew Howroyd, Sep 16 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Feb 08 2007
EXTENSIONS
a(0) and terms a(23) and beyond from Andrew Howroyd, Sep 16 2017
STATUS
approved