A139421 a(1)=1; for n>1, a(n) = largest prime divisor of n!!.

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

Offset: 1

Author

Artur Jasinski, Apr 21 2008

Keywords

• nonn

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A010693.

Programs

Mathematica

a = {}; Do[b = First[Last[FactorInteger[n!! ]]]; AppendTo[a, b], {n, 2, 100}]; a
FactorInteger[#][[-1,1]]&/@(Range[80]!!) (* Harvey P. Dale, Feb 15 2014 *)

Formula

a(n) = A006530(A006882(n)). - Michel Marcus, Nov 08 2013