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 A138866 #8 Dec 30 2023 16:52:17
%S A138866 33,77,3131,127127,819191,131071,524287,214647,230951,618111,162127,
%T A138866 170727,686151,531127,104087,147007,446351,259071,190991,285607,
%U A138866 478111,346551,281191,431471,448751,402511,854671,536207,521007,512311,746447,174887,129591,412527,814711,623151,127271,437791,924071,125047,299407,122247,315871,124871,202927,169751,316511
%N A138866 Concatenation of first 3 digits and last 3 digits of n-th Mersenne prime A000668(n).
%H A138866 L. C. Noll, <a href="http://www.isthe.com/chongo/tech/math/prime/mersenne.html">Mersenne Prime Digits and Names</a>.
%t A138866 Join[{33,77,3131},FromDigits[Flatten[Join[{Take[IntegerDigits[#],3],Take[ IntegerDigits[ #],-3]}]]]&/@ (2^MersennePrimeExponent[Range[4,40]]-1)] (* _Harvey P. Dale_, Dec 30 2023 *)
%Y A138866 Cf. A000668, A080173, A104511, A138816, A138817, A138840, A138841.
%K A138866 base,nonn
%O A138866 1,1
%A A138866 _Omar E. Pol_, Apr 02 2008
%E A138866 More terms from _Harvey P. Dale_, Dec 30 2023