login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Least prime factor of concatenation of first n odd numbers.
2

%I #4 Mar 31 2012 13:48:25

%S 1,13,3,23,37,3,11617,5,3,135791113151719,29,3,5,11,3,

%T 135791113151719212325272931,17,3,7,13,3,131,5,3,11,

%U 25471443030907588399109,3,5,7,3,181,41,3,135791113151719212325272931333537394143454749515355575961636567,19,3,40351,5,3,7,11,3,5,57041,3,351269,11,3,135791113151719212325272931333537394143454749515355575961636567697173757779818385878991939597

%N Least prime factor of concatenation of first n odd numbers.

%H Sean A. Irvine, <a href="/A138965/b138965.txt">Table of n, a(n) for n = 1..300</a>

%H F. Smarandache, <a href="http://arXiv.org/abs/math/0604019">Sequences of numbers involved in unsolved problems, arXiv:math/0604019</a>

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

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

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

%K base,nonn

%O 1,2

%A _M. F. Hasler_, Apr 14 2008