%I #5 Oct 06 2019 18:19:41
%N Concatenations C1 and C2 are both prime (see the comment lines).
%C C1 = 'prevprime(n) followed by n'
%C C2 = 'n followed by prevprime(n)'
%e n=747 -> previous prime is 743, thus '743747' and '747743' are both primes.
%Y Cf. A034808-A034821.
%A _Patrick De Geest_, Oct 15 1998