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 A135623 #7 Feb 26 2023 13:03:12 %S A135623 3,7,13,721,1918,170131,782425,7463847412,1593963129003485032, %T A135623 111265944731096246910079816,721882010875193363312928672952261, %U A135623 727501488517303786137132964064381141071 %N A135623 Mersenne primes written backwards. %H A135623 Harvey P. Dale, <a href="/A135623/b135623.txt">Table of n, a(n) for n = 1..18</a> %H A135623 Omar E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>. %e A135623 a(3)=13 because the 3rd Mersenne prime is 31. %t A135623 IntegerReverse[2^MersennePrimeExponent[Range[15]]-1] (* _Harvey P. Dale_, Feb 26 2023 *) %Y A135623 Cf. A000668, A004087. %K A135623 base,easy,nonn %O A135623 1,1 %A A135623 _Omar E. Pol_, Feb 17 2008