OFFSET
0,2
LINKS
Robert Price and Charles R Greathouse IV, Table of n, a(n) for n = 0..10000 (first 400 terms from Price)
FORMULA
a(n) = 1 + 4n + 4*Sum_{k=1..n} tau(k), where tau(k) is the number of divisors of k. - Lorenz H. Menke, Jr., Mar 15 2016
MAPLE
with(numtheory): A226355:=n->1+4*n+4*add(tau(k), k=1..n): seq(A226355(n), n=0..100); # Wesley Ivan Hurt, Jan 10 2017
MATHEMATICA
f[n_] := Length[Complement[Union[Flatten[Table[If[Abs[i]*Abs[j] ≤ n, {i, j}], {i, -n, n}, {j, -n, n}], 1]], {Null}]]; Table[f[n], {n, 0, 100}]
f[n_]:=4 Sum[Length[Divisors[k]], {k, 1, n}] + 4 n + 1 (* Lorenz H. Menke, Jr., Mar 15 2016 *)
PROG
(PARI) a(n) = 8*sum(k=1, sqrtint(n), n\k) - 4*sqrtint(n)^2 + 4*n + 1 \\ Charles R Greathouse IV, Mar 16 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert Price, Jun 04 2013
STATUS
approved