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 A308721 #16 Jul 09 2019 03:03:00
%S A308721 0,1,3,20,24,10,30,276,388,81,225,726,2935,1408,7718,5624,5680,85998,
%T A308721 26706,546290,1112929,702575,3425417,10537360,21301906,217230900,
%U A308721 108698091,32381774,846522986,851764846,11692311325,46163898987
%N A308721 Locations of the first occurrence of pair (0,n) in the van Eck sequence (A181391).
%C A308721 a(33) = 118456929919 and a(34) = 250327022558 but we do not yet know a(32).
%e A308721 For a(5)=10, the pair (0,5) first occurs in A181391 at element 10.
%t A308721 With[{s = Nest[# /. {{Longest[p___], a_, q___, a_} :> {p, a, q, a, Length[{a, q}]}, {a___} :> {a, 0}} &, {}, 10^3]}, TakeWhile[#, # > -1 &] &@ Array[If[Length@ # == 0, -1, #[[1, 1]] - 1 ] &@ SequencePosition[s, {0, #}] &, Max@ s, 0]] (* _Michael De Vlieger_, Jul 08 2019, after _JungHwan Min_ at A181391 *)
%Y A308721 Cf. A181391.
%K A308721 sign,more
%O A308721 0,3
%A A308721 _Tomas Rokicki_, Jun 19 2019