OFFSET
1,4
COMMENTS
Problem: What are the positions of last appearances > 1?
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Product_{k=A001222(n)/2+1..A001222(n)} A027746(n,k) if A001222(n) is even, and 1 otherwise. - Amiram Eldar, Nov 02 2024
EXAMPLE
The divisors of 90 with half bigomega are: 6, 9, 10, 15, so a(90) = 15.
MATHEMATICA
Table[If[#=={}, 1, Max[#]]&@Select[Divisors[n], PrimeOmega[#]==PrimeOmega[n]/2&], {n, 100}]
a[n_] := Module[{p = Flatten[Table[#[[1]], {#[[2]]}] & /@ FactorInteger[n]], np}, np = Length[p]; If[OddQ[np], 1, Times @@ p[[np/2+1 ;; np]]]]; Array[a, 100] (* Amiram Eldar, Nov 02 2024 *)
PROG
(Python)
from sympy import divisors, factorint
def a(n):
npf = len(factorint(n, multiple=True))
for d in divisors(n)[-1:0:-1]:
if 2*len(factorint(d, multiple=True)) == npf: return d
return 1
print([a(n) for n in range(1, 82)]) # Michael S. Branicky, Aug 18 2021
(Python 3.8+)
from math import prod
from sympy import factorint
def A347046(n):
fs = factorint(n, multiple=True)
q, r = divmod(len(fs), 2)
return 1 if r else prod(fs[q:]) # Chai Wah Wu, Aug 20 2021
CROSSREFS
Positions of 1's are A026424.
The case of powers of 2 is A072345.
The rounded version is A347044.
A000005 counts divisors.
A001221 counts distinct prime factors.
A001222 counts all prime factors (also called bigomega).
A340387 lists numbers whose sum of prime indices is twice bigomega.
A340609 lists numbers whose maximum prime index divides bigomega.
A340610 lists numbers whose maximum prime index is divisible by bigomega.
A347042 counts divisors d|n such that bigomega(d) divides bigomega(n).
KEYWORD
nonn,changed
AUTHOR
Gus Wiseman, Aug 16 2021
STATUS
approved