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 A292880 #9 Sep 28 2017 19:56:59
%S A292880 1,32,633,10744,173705,2798376,45930577,777101648,13638044529,
%T A292880 249079033360,4741200949001,94104123729672,1947270419971513,
%U A292880 41985753920469464,942531024150018465,22009425078894009376,534085741053864862817,13454221423402868473728,351483652960252663137049,9512821482149972773978520
%N A292880 Number of sequences of balls colored with at most n colors such that exactly three balls are of a color seen earlier in the sequence.
%C A292880 Note that any such sequence has at least 4 balls and at most n+3.
%F A292880 a(n) = n! * Sum_{k=4..n+3} [binomial(k,4)+10*binomial(k,5)+15*binomial(k,6)]/(n+3-k)!
%Y A292880 Row sums of triangle A292879.
%K A292880 nonn
%O A292880 1,2
%A A292880 _Jeremy Dover_, Sep 25 2017