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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

Table of n, a(n) for n=1..20.

FORMULA

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

CROSSREFS

Row sums of triangle A292879.

Sequence in context: A013699 A283279 A004337 * A255262 A181240 A140802

Adjacent sequences:  A292877 A292878 A292879 * A292881 A292882 A292883

KEYWORD

nonn

AUTHOR

Jeremy Dover, Sep 25 2017

STATUS

approved

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Last modified June 6 15:37 EDT 2020. Contains 334828 sequences. (Running on oeis4.)