A090261 - OEIS

%I #9 Jun 10 2018 18:15:58

%S 1,3,1,9,1,17,1,21,11,13,17,7,3,11,19,9,7,111,17,13,23,1,27,17,37,3,

%T 13,33,23,7,129,11,27,43,3,17,39,17,103,3,19,17,121,117,1,29,1,39,23,

%U 9,13,51,1,41,3,37,21,13,53,1,47,1,51,7,131,7,137,19,23,13,71,7,149,1,59,3

%N Beginning with a(1) = 1, choose a(n+1) so that concatenation a(n+1), a(n), a(n+1) is a new prime.

%H H. v. Eitzen, <a href="/A090261/b090261.txt">Table of n, a(n) for n = 1..10000</a>.

%e 313, 131, 919, 191, 17117, 1171, 21121, etc. are primes.

%o (PARI) tp(n)=10^ceil(log(n+0.5)/log(10));seen=Set([]);a=[1]; while(#a<100,n=a[ #a];tn=tp(n);k=1;while(!isprime(z=(k*tn+n)*tp(k)+k) || setsearch(seen,z), k+=2);seen=setunion(seen,[z]);a=concat(a,k));a \\ _Hagen von Eitzen_, Jun 21 2009

%Y Cf. A090260.

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Nov 28 2003

%E a(14) corrected and more terms added by _Hagen von Eitzen_, Jun 21 2009