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 A321032 Number of words of length 3n such that all letters of the binary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting triples into the initially empty word. 2
 3, 18, 97, 530, 2973, 17059, 99657, 590562, 3540463, 21430266, 130771375, 803538099, 4967127735, 30866224823, 192696614729, 1207967820098, 7600482116931, 47981452358200, 303820299643137, 1929099000980218, 12279621792772821, 78346444891033855 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 LINKS Alois P. Heinz, Table of n, a(n) for n = 2..1211 EXAMPLE a(2) = 3: aaabbb, aabbba, abbbaa. a(3) = 18: aaaaaabbb, aaaaabbba, aaaabbbaa, aaabaaabb, aaabbaaab, aaabbbaaa, aaabbbbbb, aabaaabba, aabbaaaba, aabbbaaaa, aabbbabbb, aabbbbbba, abaaabbaa, abbaaabaa, abbbaaaaa, abbbaabbb, abbbabbba, abbbbbbaa. 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))(2): seq(a(n), n=2..25); CROSSREFS Column k=2 of A256311. Sequence in context: A129547 A081151 A132848 * A180036 A038158 A327828 Adjacent sequences:  A321029 A321030 A321031 * A321033 A321034 A321035 KEYWORD nonn AUTHOR Alois P. Heinz, Oct 26 2018 STATUS approved

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Last modified September 17 06:16 EDT 2021. Contains 347478 sequences. (Running on oeis4.)