OFFSET
1,4
COMMENTS
A permutation pi is a forbidden pattern if there is no x in [0,1] such that the values x,f(x),f(f(x)),...,f^{n-1}(x) are in the same relative order as pi_1,pi_2,...,pi_n. A forbidden pattern is basic if it is minimally forbidden, that is, the patterns obtained by removing pi_1 or pi_n are not forbidden.
a(n) is also the number of basic forbidden patterns of length n of the tent map x -> 1-|1-2x| in [0,1].
LINKS
S. Elizalde and Y. Liu, On basic forbidden patterns of functions, arXiv:0909.2277 [math.CO], 2009.
S. Elizalde and Y. Liu, On basic forbidden patterns of functions, Discrete Appl. Math. 159 (2011), 1207-1216.
EXAMPLE
a(3) = 1 because the only basic forbidden pattern of length 3 is 321.
a(4) = 5 because the basic forbidden patterns of length 4 are 1423, 2134, 2143, 3142, 4231.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Sergi Elizalde, Jul 22 2011
STATUS
approved