OFFSET
1,1
COMMENTS
Also numbers n whose median prime factor is not a prime factor of n, where the median of a multiset is either the middle part (for odd length), or the average of the two middle parts (for even length).
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
The prime factorization of 60 is 2*2*3*5, with middle parts (2,3), so 60 is in the sequence.
MAPLE
filter:= proc(n) local F, m;
F:= sort(map(t -> t[1]$t[2], ifactors(n)[2]));
m:= nops(F);
m::even and F[m/2] <> F[m/2+1]
end proc:
select(filter, [$2..200]); # Robert Israel, Dec 15 2023
MATHEMATICA
prifacs[n_]:=If[n==1, {}, Flatten[ConstantArray@@@FactorInteger[n]]];
Select[Range[2, 100], FreeQ[prifacs[#], Median[prifacs[#]]]&]
KEYWORD
nonn
AUTHOR
Gus Wiseman, May 10 2023
STATUS
approved