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 A138841 #11 Apr 19 2018 12:36:09 %S A138841 33,77,31,17,81,11,57,27,21,61,17,17,61,57,17,17,41,21,11,27,41,31,21, %T A138841 41,41,41,81,57,57,51,77,17,11,47,81,61,11,41,91,17,27,17,31,11,27,11, %U A138841 31 %N A138841 Concatenation of initial and final digit of n-th Mersenne prime A000668(n). %F A138841 a(n) = A073729(A000668(n)). - _Michel Marcus_, Apr 17 2018 %e A138841 a(5)=81 because the 5th Mersenne prime is 8191, A000668(5)=8191. %Y A138841 Cf. A000668, A073729, A080172, A135613, A138840, A138842, A138843. %K A138841 base,nonn,more,less %O A138841 1,1 %A A138841 _Omar E. Pol_, Apr 01 2008 %E A138841 a(40)-a(47) from _Ivan Panchenko_, Apr 17 2018