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A321037
Number of words of length 3n such that all letters of the septenary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting triples of identical letters into the initially empty word.
2
7752, 942172, 69858360, 4110110550, 211675627240, 10018481786544, 448004505210535, 19256258184412314, 804574034878107648, 32931600573236529164, 1327621591399983233480, 52924527085674622431690, 2092267882428782767625532, 82204203906811813227807616
OFFSET
7,1
LINKS
MAPLE
b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(7):
seq(a(n), n=7..25);
CROSSREFS
Column k=7 of A256311.
Sequence in context: A031586 A116335 A115440 * A031646 A252556 A028538
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 26 2018
STATUS
approved