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 A138862 #11 Aug 01 2020 17:54:24
%S A138862 3,7,31,12,81,13,52,21,23,61,16,17,68,53,10,14,44,25,19,28,47,34,28,
%T A138862 43,44,40,85,53,52,51,74,17,12,41,81,62,12,43,92,12,29,12,31,12,20,16,
%U A138862 31
%N A138862 First two (i.e., the two most significant) digits of n-th Mersenne prime A000668(n).
%H A138862 L. C. Noll, <a href="http://www.isthe.com/chongo/tech/math/prime/mersenne.html">Mersenne Prime Digits and Names</a>.
%t A138862 Join[{3,7},FromDigits[Take[IntegerDigits[#],2]]&/@ (2^MersennePrimeExponent[ Range[ 3,47]]-1)] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 01 2020 *)
%Y A138862 Cf. A000668, A080173, A138863, A138864, A138865, A138866.
%K A138862 base,nonn
%O A138862 1,1
%A A138862 _Omar E. Pol_, Apr 02 2008
%E A138862 a(40)-a(47) from _Ivan Panchenko_, Aug 03 2018
%E A138862 Definition clarified by _Harvey P. Dale_, Aug 01 2020