

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.


0



1, 32, 633, 10744, 173705, 2798376, 45930577, 777101648, 13638044529, 249079033360, 4741200949001, 94104123729672, 1947270419971513, 41985753920469464, 942531024150018465, 22009425078894009376, 534085741053864862817, 13454221423402868473728, 351483652960252663137049, 9512821482149972773978520
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OFFSET

1,2


COMMENTS

Note that any such sequence has at least 4 balls and at most n+3.


LINKS



FORMULA

a(n) = n! * Sum_{k=4..n+3} [binomial(k,4)+10*binomial(k,5)+15*binomial(k,6)]/(n+3k)!


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



