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 A117823 #6 Sep 03 2017 11:27:16
%S A117823 0,1,3,7,6,8,20,21,33,45,68,102,158,247,391,623,998,1604,2584,4169,
%T A117823 6733,10881,17592,28450,46018,74443,120435,194851,315258,510080,
%U A117823 825308,1335357,2160633,3495957,5656556,9152478,14808998,23961439,38770399
%N A117823 a(0) = 0, a(1) = 1; for n >= 2, a(n) = a(n-1) + a(n-2) - n if that number is positive and not already in the sequence, otherwise a(n) = a(n-1) + a(n-2) + n.
%p A117823 a[0]:=0: a[1]:=1: A:={0,1}: for n from 2 to 42 do if a[n-1]+a[n-2]-n>0 and member(a[n-1]+a[n-2]-n,A)=false then a[n]:=a[n-1]+a[n-2]-n: A:=A union {a[n-1]+a[n-2]-n} else a[n]:=a[n-1]+a[n-2]+n: A:=A union {a[n-1]+a[n-2]+n} fi: od: seq(a[n],n=0..42); # _Emeric Deutsch_, Apr 30 2006
%Y A117823 Cf. A005132, A079053, A117821, A117822, A117824.
%K A117823 nonn,easy
%O A117823 0,3
%A A117823 _N. J. A. Sloane_, Apr 30 2006
%E A117823 More terms from _Emeric Deutsch_, Apr 30 2006