OFFSET
1,1
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
Michael De Vlieger, Log log scatterplot of a(n), n = 1..2^20.
FORMULA
EXAMPLE
Let b(n) = A120944(n).
a(1) = 3 since b(1) = 6, and 3 is the second smallest prime factor.
a(2) = 5 since b(2) = 10, and 5 is the second smallest prime factor.
Table showing select values of a(n):
n b(n) a(n)
-----------------------
1 6 = 2*3 3
2 10 = 2*5 5
3 14 = 2*7 7
4 15 = 3*5 5
5 21 = 3*7 7
6 22 = 2*11 11
7 26 = 2*13 13
8 30 = 2*3*5 3
14 42 = 2*3*7 3
22 66 = 2*3*11 3
24 70 = 2*5*7 5
82 210 = 2*3*5*7 3
MATHEMATICA
Map[FactorInteger[#][[2, 1]] &, Select[Range[250], And[SquareFreeQ[#], CompositeQ[#]] &]]
PROG
(Python)
from math import isqrt
from sympy import primepi, mobius, primefactors
def A376833(n):
def f(x): return n+1+primepi(x)+x-sum(mobius(k)*(x//k**2) for k in range(1, isqrt(x)+1))
m, k = n+1, f(n+1)
while m != k:
m, k = k, f(k)
return primefactors(m)[1] # Chai Wah Wu, Oct 06 2024
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Michael De Vlieger, Oct 05 2024
STATUS
approved