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%I #10 Aug 23 2014 15:08:28
%S 1,1,0,1,7,3,1,0,3,7,13,11,0,1,27,1,7,0,3,7,9,39,0,9,1,19,51,0,1,3,7,
%T 1,0,3,49,9,3,0,17,19,1,9,0,7,23,1,11,0,1,1,77,1,0,33,1,109,11,0,1,3,
%U 3,1,0,7,1,51,39,0,3,21,59,21,0,13,47,1,1,0,9,7,9,19,0,29,3,37,149,0,7,21
%N Smallest number k such that the concatenations k 2n-1 and 2n-1 k are both primes, or 0 if n = 5m+3.
%C Observation: Most of the members are prime or 1. The first composite k are a(15)=27, a(21)=9, a(22)=39, a(24)=9, a(27)=51, a(35)=49....
%e a(19) = 7 as 197 and 719 both are prime, a(15) =0.
%p A083803 := proc(n)
%p if modp(n,5) = 3 then
%p 0;
%p else
%p for k from 1 do
%p if isprime(digcat2(k,2*n-1) ) then
%p if isprime(digcat2(2*n-1,k)) then
%p return k;
%p end if;
%p end if;
%p end do:
%p end if;
%p end proc: # _R. J. Mathar_, Aug 23 2014
%K base,nonn
%O 1,5
%A _Amarnath Murthy_ and Meenakshi Srikanth (menakan_s(AT)yahoo.com), May 08 2003
%E More terms from _David Wasserman_, Nov 30 2004
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