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.

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

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