OFFSET
1,1
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
Charles Kusniec, Composite Numbers Map for the OEIS.
Charles Kusniec, Distribution of gaps in A392008 for the first 4350 terms
EXAMPLE
The smallest odd squarefree number with exactly 3 distinct prime factors is 105 = 3*5*7.
MAPLE
q:= n-> (l-> nops(l)>2 and {l[]}={1})(ifactors(n)[2][.., 2]):
select(q, [2*i+1$i=0..577])[]; # Alois P. Heinz, Dec 29 2025
MATHEMATICA
Select[Range[1, 1160, 2], SquareFreeQ[#] && PrimeOmega[#] >= 3 &] (* Michael De Vlieger, Dec 29 2025 *)
PROG
(Python)
from math import isqrt, prod
from sympy import primerange, integer_nthroot, primepi
from oeis_sequences.OEISsequences import bisection
def A392008(n):
def g(x, a, b, c, m): yield from (((d, ) for d in enumerate(primerange(b+1, isqrt(x//c)+1), a+1)) if m==2 else (((a2, b2), )+d for a2, b2 in enumerate(primerange(b+1, integer_nthroot(x//c, m)[0]+1), a+1) for d in g(x, a2, b2, c*b2, m-1)))
def f(x): return int(n+x-sum(sum(primepi(x//prod(c[1] for c in a))-a[-1][0] for a in g(x, 1, 2, 1, i)) for i in range(3, x.bit_length())))
return bisection(f, n, n) # Chai Wah Wu, Jan 07 2026
CROSSREFS
KEYWORD
nonn
AUTHOR
Charles Kusniec, Dec 26 2025
STATUS
approved
