OFFSET
1,4
COMMENTS
Permuting the symbols will not change the structure.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..1000
EXAMPLE
The a(3) = 1 word is 123.
The a(4) = 3 words are 1123, 1223, 1233.
The a(5) = 12 words are 11123, 11213, 11223, 11231, 11233, 12113, 12223, 12232, 12233, 12311, 12322, 12333.
PROG
(PARI) \\ See A351641 for R, S.
seq(n)={my(q=S(n), c=3); sum(k=1, c, R(q^k-1)*binomial(c, k)*(-1)^(3-k))/c!}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Feb 16 2022
STATUS
approved