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%I #5 Oct 06 2019 18:19:41
%S 9,51,63,87,111,123,153,177,207,211,239,263,273,289,327,333,343,359,
%T 407,471,489,497,513,541,597,621,651,659,663,681,687,693,697,747,753,
%U 793,819,831,869,909,977,987,1027,1041,1089,1131,1143,1239,1491,1611
%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.
%K nonn,base
%O 0,1
%A _Patrick De Geest_, Oct 15 1998
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