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%I #10 Jun 22 2019 15:02:49
%S 2,1212121,12121212121,1212121212121212121212121212121212121212121
%N Primes in A028359.
%C Except for the first term, each term must both begin and end with a "1" because each term greater than 2 must end in a "1" and if a term begins with a "2" the sum of the digits of the term will be divisible by 3 and thus the term is not a prime. - _Harvey P. Dale_, Jun 22 2019
%C The next term (a(5)) has 139 digits and a(6) has 627 digits. - _Harvey P. Dale_, Jun 22 2019
%t Join[{2},Select[Table[FromDigits[PadRight[{},2n+1,{1,2}]],{n,30}],PrimeQ]] (* _Harvey P. Dale_, Jun 22 2019 *)
%Y Cf. A028359.
%K nonn
%O 1,1
%A _Zak Seidov_, Nov 28 2009
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