A242846 - OEIS

%I #31 Mar 27 2021 22:37:00

%S 131,313,1313131313131313131313131,

%T 313131313131313131313131313131313131313131313131313

%N Palindromic primes of the form ababa...aba containing only the digits 1 and 3.

%C The next two terms both start with 3 and have 83 and 225 digits, respectively. Those are the only other terms with fewer than 352 digits. Cf. A062216.

%t Module[{nn=60,a,b},a=Table[FromDigits[Join[PadRight[{},2n,{1,3}],{1}]],{n,nn}];b=Table[FromDigits[Join[PadRight[{},2n,{3,1}],{3}]],{n,nn}];Select[Sort[Join[a,b]],PrimeQ]] (* _Harvey P. Dale_, Sep 07 2020 *)

%Y Cf. A062216, A032758, A048399, A235154.

%K nonn,base

%O 1,1

%A _Felix Fröhlich_, May 23 2014