OFFSET
1,6
COMMENTS
Also, number of ordered pairs (a, b) with 0 < a <= b such that a*b = c*n + d and c = d where 0 < a, b, c, d < n. - Naohiro Nomoto, Oct 02 2021
LINKS
R. H. Hardin, Table of n, a(n) for n=1..10000
EXAMPLE
The a(12)=9 solutions for product of a single 1..11 pair == 0 (mod 12) are 2*6, 3*4, 3*8, 4*6, 4*9, 6*6, 6*8, 6*10, and 8*9.
MATHEMATICA
f1[p_, e_] := (e*(p - 1)/p + 1)*p^e; f2[p_, e_] := p^Floor[e/2]; a[n_] := (Times @@ f1 @@@ (fct = FactorInteger[n]) + Times @@ f2 @@@ fct)/2 - n; Array[a, 100] (* Amiram Eldar, Apr 28 2023 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, formula from Max Alekseyev in the Sequence Fans Mailing List, Sep 20 2010
STATUS
approved