A087040 2nd largest prime factor of n-th composite number.

2, 2, 2, 3, 2, 2, 2, 3, 2, 3, 2, 3, 2, 2, 5, 2, 3, 2, 3, 2, 3, 2, 5, 3, 2, 3, 2, 3, 2, 3, 2, 2, 7, 5, 3, 2, 3, 5, 2, 3, 2, 3, 2, 3, 2, 5, 3, 2, 3, 5, 3, 2, 5, 2, 7, 3, 2, 3, 2, 3, 5, 2, 3, 2, 3, 7, 2, 3, 2, 5, 2, 7, 3, 5, 3, 2, 5, 2, 3, 5, 3, 2, 3, 5, 2, 3, 2, 7, 3, 11, 2, 3, 2, 5, 3, 2, 3, 5, 3, 7, 2, 3

Offset

1,1

Comments

a(n) = A087039(A002808(n));

a(n) = A006530(A002808(n)/A052369(n)).

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

f:= proc(n) local F;
  if isprime(n) then return NULL fi;
  F:= sort(ifactors(n)[2], (a, b) -> a[1]>b[1]);
  if F[1][2] >= 2 then F[1][1] else F[2][1] fi
end proc:
map(f, [$2..200]); # Robert Israel, Feb 20 2024

Mathematica

Table[l=FactorInteger[ResourceFunction["Composite"][n]]; If[Last[l][[2]]>1, Last[l][[1]], First[Part[l, -2]]], {n, 102}] (* James C. McMahon, Feb 20 2024 *)

Crossrefs

Cf. A002808, A006530, A052369, A087039.

Sequence in context: A270776 A056608 A091787 * A065569 A262941 A127656

Adjacent sequences: A087037 A087038 A087039 * A087041 A087042 A087043

Keyword

nonn

Author

Reinhard Zumkeller, Aug 01 2003

Status

approved