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 A281946 #17 Sep 29 2017 07:41:18
%S A281946 1,16,195,2248,26245,318936,4082071,55289200,793525833,12063384640,
%T A281946 194002619371,3294811981176,58980720557005,1110692723476168,
%U A281946 21960340413007935,455018383693865056,9862401602086024081,223233406292824965360,5268151612376938762003,129425572759622914323880
%N A281946 Number of sequences of balls colored with at most n colors such that exactly two balls are of a color seen earlier in the sequence.
%C A281946 Note that any such sequence has at least 3 balls and at most n+2.
%H A281946 Jeremy Dover, <a href="/A281946/b281946.txt">Table of n, a(n) for n = 1..100</a>
%F A281946 a(n) = n! * Sum_{k=3..n+2} (binomial(k,3)+3*binomial(k,4))/(n+2-k)!.
%F A281946 a(n)/n! ~ e*n^4/8. - _Vaclav Kotesovec_, Feb 03 2017
%t A281946 Table[n! * Sum[(Binomial[k,3]+3*Binomial[k,4])/(n+2-k)!, {k, 3, n+2}], {n, 1, 20}] (* _Vaclav Kotesovec_, Feb 03 2017 *)
%Y A281946 Row sums of triangle A281944.
%K A281946 nonn
%O A281946 1,2
%A A281946 _Jeremy Dover_, Feb 02 2017