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 A138833 #10 Jan 10 2023 13:12:19
%S A138833 63,287,49631,8128127,335503368191,8589869056131071,
%T A138833 137438691328524287,23058430081399521282147483647,
%U A138833 26584559915698317446546926159538421762305843009213693951,191561942608236107294793378084303638130997321548169216618970019642690137449562111
%N A138833 Concatenation of n-th perfect number and n-th Mersenne prime A000668(n).
%C A138833 a(15) has 1156 digits. - _Harvey P. Dale_, Jan 10 2023
%H A138833 Harvey P. Dale, <a href="/A138833/b138833.txt">Table of n, a(n) for n = 1..14</a>
%e A138833 a(3)=49631 because A000396(3)=496 and A000668(3)=31.
%t A138833 Module[{nn=10},FromDigits[Flatten[IntegerDigits/@#]]&/@Thread[{PerfectNumber[ Range[ nn]],2^ MersennePrimeExponent[ Range[nn]]-1}]] (* _Harvey P. Dale_, Jan 10 2023 *)
%Y A138833 Cf. A000396, A000668, A138816, A138817, A138818, A138819, A138820, A138821, A138822, A138832.
%K A138833 base,nonn
%O A138833 1,1
%A A138833 _Omar E. Pol_, Apr 06 2008
%E A138833 Corrected and extended by _Harvey P. Dale_, Jan 10 2023