%I #32 Jul 22 2024 05:42:06
%S 2,23,5,2357,7,23,11,7,61,3,17,7,25391,6899,1597,3,17,3,1019,3,17,211,
%T 41378209,3,479,3,262343078386137703,3,107,3,643,3,7,3,13,3,107069,7,
%U 14733071537,3,61911427,3,13,3,31,3,11,59,7,647,31,3,13,3,31517,43,3,19
%N a(n) is the smallest prime divisor of the number obtained from concatenation of the first n primes.
%H Sean A. Irvine, <a href="/A104644/b104644.txt">Table of n, a(n) for n = 1..113</a>
%H Dario Alejandro Alpern, <a href="https://www.alpertron.com.ar/ECM.HTM">Factorization using the Elliptic Curve Method</a>.
%H Hisanori Mishima, <a href="https://www.asahi-net.or.jp/~KC2H-MSM/mathland/matha1/sm_prime.htm">Smarandache consecutive prime sequences (n = 1 to 100)</a>.
%F a(n) = A020639(A019518(n)). - _Michel Marcus_, Aug 01 2017
%e n=5, concatenation of {2,3,5,7,11} is 235711=7*151*223, smallest prime factor is a(5)=7.
%t Map[FactorInteger[FromDigits@ #][[1, 1]] &, Rest@ FoldList[Join @@ {#1, IntegerDigits[#2]} &, {}, Prime@ Range@ 29]] (* _Michael De Vlieger_, Jul 31 2017 *)
%o (PARI) a(n) = vecmin(factor(eval(concat(concat([""], primes(n)))))[,1]); \\ _Michel Marcus_, Aug 01 2017
%Y Cf. A019518, A020639, A074809.
%K nonn,base
%O 1,1
%A _Labos Elemer_, Mar 18 2005
%E a(29) and further terms from _Robert G. Wilson v_, Mar 18 2005
%E Edited by _Charles R Greathouse IV_, Apr 23 2010