OFFSET
1,10
COMMENTS
Division by six is performed so that strings that are identical up to swapping the symbols are not double-counted.
The corresponding sequence for strings of length n on two symbols is given by a(n) = 2^(n-1) - n = A000295(n-1).
LINKS
Paul Tek, Table of n, a(n) for n = 1..2000
Paul Tek, PERL program for this sequence
EXAMPLE
a(9) = 1 because there are 6 strings of length 9 on the three symbols "1", "2", and "3" containing each of "123", "132", "213", "231", "312", and "321" as substrings: they are "123121321" and the five other strings obtained by swapping the roles of "1", "2", and "3" in that string.
The substrings must be contiguous -- if they were allowed to be non-contiguous (i.e., subsequences) then there would be a valid string of length 7: "1232132" (see A062714).
CROSSREFS
KEYWORD
nonn
AUTHOR
Nathaniel Johnston, Mar 30 2011
STATUS
approved