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 A138828 #25 Feb 07 2019 13:31:08
%S A138828 6,10,19,19,28,64,55,73,190,235,289,352,1405,1711,3520,5833,5968,8821,
%T A138828 11548,11791,26317,27298,30232,53740,58960,62956,120898,233722,299314,
%U A138828 356860,585478,2048248,2329372,3405232,3789352,8056495,8186041,18894079,36485416,56880973,65115946,70334902,82384129
%N A138828 Sum of digits of n-th even perfect number.
%H A138828 Ivan Panchenko, <a href="/A138828/b138828.txt">Table of n, a(n) for n = 1..47</a>
%H A138828 Omar E. Pol, <a href="http://www.polprimos.com">Determinacion geometrica de los numeros primos y perfectos</a>.
%F A138828 a(n) = A007953(A000396(n)). - _R. J. Mathar_, May 22 2008 [This assumes that all perfect numbers are even. - _Ivan Panchenko_, Aug 16 2018]
%t A138828 a[n_] := Block[{e, p, mpe = MersennePrimeExponent@n}, Plus @@ IntegerDigits[(2^mpe - 1) 2^(mpe - 1)]]; Array[a, 44] (* _Robert G. Wilson v_, Aug 06 2018 *)
%t A138828 Total[IntegerDigits[#]]&/@Table[PerfectNumber[n],{n,43}] (* Requires Mathematica version 10 or later *) (* The program may take a long time to run *) (* _Harvey P. Dale_, Feb 07 2019 *)
%Y A138828 Cf. A000396, A007605, A066538, A138127.
%K A138828 base,nonn
%O A138828 1,1
%A A138828 _Omar E. Pol_, Apr 01 2008
%E A138828 More terms from _R. J. Mathar_, May 22 2008
%E A138828 a(15)-a(38) from _Donovan Johnson_, Nov 09 2010
%E A138828 Definition changed (inserting the word "even") and a(39)-a(43) added by _Ivan Panchenko_, Aug 06 2018