OFFSET
1,3
COMMENTS
Contains all positive integers except 1, 2, 4.
FORMULA
a(2n) = a(n).
Completely additive with a(2^e) = 0 and a(p^e) = e*p for an odd prime p. - Amiram Eldar, Nov 03 2023
EXAMPLE
The prime factors of 60 are {2,2,2,3,5}, of which the odd factors are {3,5}, so a(60) = 8.
MATHEMATICA
Table[Total[Times@@@DeleteCases[If[n==1, {}, FactorInteger[n]], {2, _}]], {n, 100}]
PROG
(PARI) a(n) = my(f=factor(n), j=if(n%2, 1, 2)); sum(i=j, #f~, f[i, 1]*f[i, 2]); \\ Michel Marcus, Oct 30 2023
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Gus Wiseman, Oct 27 2023
STATUS
approved