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 A102403 Number of Dyck paths of semilength n having no ascents of length 2. 5
 1, 1, 1, 2, 6, 17, 46, 128, 372, 1109, 3349, 10221, 31527, 98178, 308179, 973911, 3096044, 9894393, 31770247, 102444145, 331594081, 1077022622, 3509197080, 11466710630, 37567784437, 123380796192, 406120349756, 1339571374103 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Number of Lukasiewicz paths of length n having no (1,1) steps. A Lukasiewicz path of length n is a path in the first quadrant from (0,0) to (n,0) using rise steps (1,k) for any positive integer k, level steps (1,0) and fall steps (1,-1) (see R. P. Stanley, Enumerative Combinatorics, Vol. 2, Cambridge Univ. Press, Cambridge, 1999, p. 223, Exercise 6.19w; the integers are the slopes of the steps). Example: a(4)=6 because we have HHHH, HU(2)DD, U(2)DDH, U(2)DHD, U(2)HDD and U(3)DDD where H=(1,0), U(2)=(1,2), U(3)=(1,3) and D=(1,-1). LINKS FORMULA G.f.=G=G(z) satisfies z^3*G^3+z(1-z)G^2-G+1=0. a(n)=1/n*sum(j=ceiling((n+2)/3)..n, binomial(n,j)*binomial(3*j-n-2,j-1)*(-1)^(n-j)), n>0. [From Vladimir Kruchinin, Mar 07 2011]. a(n) is the upper left term in M^n, M = an infinite square production matrix as follows: 1, 1, 0, 0, 0, 0,... 0, 1, 1, 0, 0, 0,... 1, 0, 1, 1, 0, 0,... 1, 1, 0, 1, 1, 0,... 1, 1, 1, 0, 1, 1,... - Gary W. Adamson, Jan 30 2012 EXAMPLE a(3)=2 because we have UDUDUD and UUUDDD, where U=(1,1) and D=(1,-1); the other three Dyck paths of semilength 3, namely UD(UU)DD, (UU)DDUD and (UU)DUDD, have ascents of length 2 (shown between parentheses). MAPLE Order:=35: S:=solve(series(V*(1-V)/(1-V^2+V^3), V)=z, V): seq(coeff(S, z^n), n=1..33); # V=zG PROG (Maxima) a102403(n):=1/n*sum(binomial(n, j)*binomial(3*j-n-2, j-1)*(-1)^(n-j), j, ceiling((n+2)/3), n); [From Vladimir Kruchinin, Mar 07 2011]. CROSSREFS Cf. A102402. Sequence in context: A109961 A190050 A005592 * A200379 A032638 A090039 Adjacent sequences:  A102400 A102401 A102402 * A102404 A102405 A102406 KEYWORD nonn AUTHOR Emeric Deutsch, Jan 06 2005 STATUS approved

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