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%I #8 Apr 02 2020 19:29:40
%S 3,5,21,69,313,699,798,989,3904,7029
%N Numbers n such that 12 repeated n times followed by 1 is a palindromic prime.
%H P. De Geest, <a href="http://www.worldofnumbers.com/undulat.htm">SUPP Reference Table</a>
%e 12121212121 is prime so 5 appears in the sequence.
%t Do[m = n; If[PrimeQ[120(10^(2n) - 1)/99 + 1], Print[n]], {n, 1, 600}]
%t (IntegerLength[#]-1)/2&/@Select[10#+1&/@Table[FromDigits[Flatten[ IntegerDigits/@ PadRight[{},n,{1,2}]]],{n,2,15000,2}],PrimeQ] (* _Harvey P. Dale_, Apr 02 2020 *)
%Y Corresponding primes are given in A092696. Corresponding decimal digit lengths are given in A062209. a(k) = (A062209(k-1)-1)/2.
%K nonn,base
%O 1,1
%A _Robert G. Wilson v_, Aug 22 2000
%E More terms from _Rick L. Shepherd_, Mar 04 2004
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