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 A056728 #12 Jan 17 2019 13:44:05
%S A056728 101,131,151,181,191,313,353,373,383,727,757,787,797,919,929,13331,
%T A056728 15551,16661,19991,72227,75557,76667,78887,79997,1333331,1444441,
%U A056728 1777771,3222223,3444443,7666667,9222229,9888889
%N A056728 Palindromic primes using only two distinct digits and only the exterior digit is different.
%C A056728 Primes of the form a*(10^c + 1) + b*(10^c - 10)/9 for 1<=a<=9, 0<=b<=9, c >= 2, a <> b. - _Robert Israel_, Nov 02 2014
%H A056728 Robert Israel, <a href="/A056728/b056728.txt">Table of n, a(n) for n = 1..127</a>
%H A056728 Patrick De Geest, <a href="http://www.worldofnumbers.com/em_deplat.htm">PDP Reference Table</a>.
%H A056728 Makoto Kamada, <a href="https://stdkmd.net/nrr/abbba.htm">Plateau and depression numbers of the form ABB...BBA</a>.
%p A056728 f:= (a,b,n) -> a*(10^n + 1) + b*(10^n - 10)/9:
%p A056728 select(isprime,[seq(seq(seq(f(a,b,n),b={$0..9} minus {a}),a=1..9),n=2..8)]);
%p A056728 # _Robert Israel_, Nov 02 2014
%t A056728 Select[Union[Flatten[Table[FromDigits[Join[{a},PadRight[{},n,b],{a}]],{n,1,7,2},{b,0,9},{a,{1,3,7,9}}]]],PrimeQ] (* _Harvey P. Dale_, Mar 07 2015 *)
%K A056728 nonn,base
%O A056728 1,1
%A A056728 _Robert G. Wilson v_, Aug 11 2000
%E A056728 Links added by _Patrick De Geest_, Nov 02 2014