A079068 Largest prime less than greatest prime factor of n but not dividing n, or 1 if no such prime exists.

1, 1, 2, 1, 3, 1, 5, 1, 2, 3, 7, 1, 11, 5, 2, 1, 13, 1, 17, 3, 5, 7, 19, 1, 3, 11, 2, 5, 23, 1, 29, 1, 7, 13, 3, 1, 31, 17, 11, 3, 37, 5, 41, 7, 2, 19, 43, 1, 5, 3, 13, 11, 47, 1, 7, 5, 17, 23, 53, 1, 59, 29, 5, 1, 11, 7, 61, 13, 19, 3, 67, 1, 71, 31, 2, 17, 5, 11, 73, 3, 2, 37, 79, 5, 13, 41, 23

Offset: 1

Author

Reinhard Zumkeller, Dec 20 2002

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 1..20000

Crossrefs

Cf. A079067, A006530.

Programs

Maple

a:= proc(n) local p, s; s:= numtheory[factorset](n);
      if s={} then return 1
    else p:= max(s);
         do if p=2 then return 1 else p:= prevprime(p) fi;
            if not p in s then return p fi
         od
      fi
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Sep 03 2019

Mathematica

a[n_] := Module[{p}, For[p = NextPrime[FactorInteger[n][[-1, 1]], -1], p>1, p = NextPrime[p, -1], If[!Divisible[n, p], Return[p]]]; 1];
Array[a, 100] (* Jean-François Alcover, Nov 04 2020 *)