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 A138831 #5 Mar 11 2014 01:34:11 %S A138831 101,11011,111101111,1111110111111,1111111111110111111111111, %T A138831 111111111111111101111111111111111, %U A138831 1111111111111111110111111111111111111 %N A138831 n-th perfect number minus 1, written in base 2. %C A138831 Subset of A138148, cyclops numbers with binary digits, only. %C A138831 Subset of A002113, palindromes in base 10. %C A138831 a(n) has 2*A090748(n) digits 1. %C A138831 The number of digits of a(n) is 2*A000043(n)-1, equal to A133033(n), the number of proper divisors of n-th perfect number. %C A138831 a(n) = (A135627(n) written in base 2). %H A138831 Omar E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>. %F A138831 a(n) = A138148(A090748(n)). %e A138831 n ... A000396(n) - 1 = A135627(n) ............. a(n) %e A138831 1 ............ 6 - 1 = ...... 5 ............... 101 %e A138831 2 ........... 28 - 1 = ..... 27 .............. 11011 %e A138831 3 .......... 496 - 1 = .... 495 ............ 111101111 %e A138831 4 ......... 8128 - 1 = ... 8127 .......... 1111110111111 %e A138831 5 ..... 33550336 - 1 = 33550335 .... 1111111111110111111111111 %Y A138831 Cf. A000043, A000396, A090748, A133033, A134808, A135627, A138131, A138148. %K A138831 nonn,base,less %O A138831 1,1 %A A138831 _Omar E. Pol_, Apr 08 2008, Apr 09 2008, Apr 14 2008