Forbidden subsequences and Chebyshev polynomials

In (West, Discrete Math. 157 (1996) 363-374) it was shown using transfer matrices that the number [Sn(123; 3214)1 of permutations avoiding the pattems 123 and 3214 is the Fibonacci number F2, (as are also IS,(213; 1234)1 and 1S~(213;4123)1 ). We now find the transfer matrix for IS , (123;r , r 1 . . . . . 2,1,r + 1)1, IS,(213;1,2 . . . . . r , r + 1)1, and ISn(213;r + 1,1,2 . . . . . r)l, determine its characteristic polynomial in terms of the Chebyshev polynomials, and go on to determine the generating function as a quotient of modified Chebyshev polynomials. This leads to an asymptotic result for each r which collapses to the exact results 2 n when r = 2 and F2, when r = 3 and to the Catalan number c, as r ~ e~. We observe that our generating function also enumerates certain lattice paths, plane trees, and directed animals, giving hope that these areas of combinatorics can be applied to enumerating permutations with excluded subsequences. ~) 1999 Elsevier Science B.V. All rights reserved