OFFSET
1,6
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{m=1..n} Sum_{d|m, d<m} (tau(d)-1). - Alois P. Heinz, Apr 09 2020
EXAMPLE
The a(4) = 1 triple is (1,2,4).
The a(8) = 6 triples are (1,2,4), (1,2,6), (1,2,8), (1,3,6), (1,4,8), (2,4,8).
MAPLE
with(numtheory):
a:= proc(n) option remember; `if`(n=0, 0, a(n-1)+
add(tau(d)-1, d=divisors(n) minus {n}))
end:
seq(a(n), n=1..80); # Alois P. Heinz, Apr 09 2020
MATHEMATICA
a[n_] := a[n] = If[n == 0, 0, a[n - 1] + Sum[DivisorSigma[0, d] - 1, {d, Most @ Divisors[n]}]];
Array[a, 80] (* Jean-François Alcover, Nov 27 2020, after Alois P. Heinz *)
PROG
(Python) an = len([(i, j, k) for i in range(1, n+1) for j in range(i+1, n+1) for k in range(j+1, n+1) if j%i==0 and k%j==0])
CROSSREFS
KEYWORD
nonn
AUTHOR
Derek Lim, Apr 08 2020
STATUS
approved