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%I #5 Oct 06 2019 18:19:02
%S 9,51,63,117,123,157,183,213,217,219,257,263,321,327,333,423,441,473,
%T 541,597,621,687,693,723,777,879,909,987,1101,1143,1187,1387,1459,
%U 1551,1887,2127,2179,2193,2817,3197,3339,3513,3633,4029,4209,4429,4529,4557
%N Concatenations C1 and C2 are both prime (see the comment lines).
%C C1 = 'prevprime(n) followed by n'
%C C2 = 'n followed by nextprime(n)'
%e n=1551 -> previous prime is 1549, next prime is 1553, thus '15491551' and '15511553' are both primes.
%Y Cf. A034808-A034821.
%K nonn,base
%O 0,1
%A _Patrick De Geest_, Oct 15 1998
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