A138965 Least prime factor of concatenation of first n odd numbers.

1, 13, 3, 23, 37, 3, 11617, 5, 3, 135791113151719, 29, 3, 5, 11, 3, 135791113151719212325272931, 17, 3, 7, 13, 3, 131, 5, 3, 11, 25471443030907588399109, 3, 5, 7, 3, 181, 41, 3, 135791113151719212325272931333537394143454749515355575961636567, 19, 3, 40351, 5, 3, 7, 11, 3, 5, 57041, 3, 351269, 11, 3, 135791113151719212325272931333537394143454749515355575961636567697173757779818385878991939597

Offset: 1

Author

M. F. Hasler, Apr 14 2008

Keywords

• base
• nonn

Links

• Sean A. Irvine, Table of n, a(n) for n = 1..300
• F. Smarandache, Sequences of numbers involved in unsolved problems, arXiv:math/0604019

Crossrefs

Cf. A019519, A046036, A048847; A109837, A104644.

Programs

PARI

t=1; for( n=2,99, print1( factor( eval( t=Str( t,2*n-1 )))[1,1], ", "))

Formula

A138965(n) = A020639(A019519(n)) (= 3 if n = 0 (mod 3)).