This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A056803 #18 Dec 11 2024 17:56:48
%S A056803 3,5,21,69,313,699,798,989,3904,7029,23249
%N A056803 Numbers k such that k copies of 12 followed by 1 is a palindromic prime.
%H A056803 P. De Geest, <a href="https://www.worldofnumbers.com/undulat.htm">SUPP Reference Table</a>
%e A056803 12121212121 is prime so 5 is a term.
%t A056803 Do[m = n; If[PrimeQ[120(10^(2n) - 1)/99 + 1], Print[n]], {n, 1, 600}]
%t A056803 (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 A056803 Corresponding primes are given in A092696. Corresponding decimal digit lengths are given in A062209. a(k) = (A062209(k-1)-1)/2.
%K A056803 nonn,base
%O A056803 1,1
%A A056803 _Robert G. Wilson v_, Aug 22 2000
%E A056803 More terms from _Rick L. Shepherd_, Mar 04 2004
%E A056803 Definition clarified by _N. J. A. Sloane_, Nov 09 2024
%E A056803 a(11) from _Michael S. Branicky_, Dec 11 2024