OFFSET
0,2
FORMULA
EXAMPLE
a(0) = 1 from (i,j) = (0,0).
a(1) = 3 from (i,j) = (0,0), (0,1), (0,-1).
a(2) = 11: (0,0), (0,+-1), (0,+-2), (+-2, 0), (+-2,+-1).
MATHEMATICA
a[n_]:=1+2n+2Floor[n/2]+4Sum[Floor[n/(2k)], {k, 1, Floor[n/2]}]
PROG
(PARI) a(n) = {my(nb = 0); for (i=-n, n, if ((i % 2) == 0, for(j=-n, n, if (abs(i*j) <= n, nb++); ); ); ); nb; }
CROSSREFS
KEYWORD
nonn
AUTHOR
Lorenz H. Menke, Jr., Mar 18 2016
EXTENSIONS
a(44) = 429 corrected by Georg Fischer, Sep 13 2023
STATUS
approved