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 A123703 #10 Oct 11 2019 12:31:19
%S A123703 1,1,1,2,3,2,2,12,264,75744,5758891776,33165272341980979200,
%T A123703 1099935289708766240667530888404210286592,
%U A123703 1209857641546707521062997518664689369471121315619387834957574646785625397657600
%N A123703 a(1)=a(2)=1. For n >= 3, a(n) = |(product{k=1 to n-1} a(k)) - (sum{j=1 to n-1} a(j))|.
%t A123703 f[l_List] := Append[l, Abs[Times @@ l - Plus @@ l]];Nest[f, {1, 1}, 12] (* _Ray Chandler_, Oct 09 2006 *)
%Y A123703 Cf. A123702.
%K A123703 easy,nonn
%O A123703 1,4
%A A123703 _Leroy Quet_, Oct 08 2006
%E A123703 Extended by _Ray Chandler_, Oct 09 2006