

A191317


Number of dispersed Dyck paths of length n (i.e., Motzkin paths of length n with no (1,0)steps at positive heights) having no UDU's, where U=(1,1) and D=(1,1).


1



1, 1, 2, 3, 5, 8, 14, 23, 40, 67, 117, 198, 346, 590, 1032, 1769, 3096, 5328, 9329, 16103, 28205, 48801, 85500, 148216, 259733, 450952, 790387, 1374044, 2408653, 4191814, 7349019, 12801243, 22445281, 39127766, 68611494, 119687036, 209890344, 366348367, 642493426, 1121992447, 1967839835
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OFFSET

0,3


COMMENTS

a(n) = A191316(n,0).
Addendum Jun 18 2011: (Start)
Also the number of length n left factors of Dyck paths having no DUD's.
Also number of dispersed Dyck paths with no DUD's. Example: a(4)=5 because we have UDHH, UUDD, HUDH, HHUD, and HHHH (here H = (1,0)). (End)


LINKS

Table of n, a(n) for n=0..40.


FORMULA

G.f.: ( sqrt(12*z^23*z^4) 1+2*zz^2+2*z^3 )/ (2*z*(12*z+z^2z^3)) = 2*(1+z^2) / ( (12*z)*(1+z^2)+sqrt((1+z^2)*(13*z^2)) ) .


EXAMPLE

a(4)=5 because we have HHHH, HHUD, HUDH, UDHH, and UUDD, where U=(1,1), D=(1,1), and H=(1,0). (UDUD does not qualify.)
a(4)=5 because we have UDUU, UUDD, UUDU, UUUD, and UUUU (UDUD does not qualify).


MAPLE

g := ((sqrt(12*z^23*z^4)1+2*zz^2+2*z^3)*1/2)/(z*(12*z+z^2z^3)): gser := series(g, z = 0, 45): seq(coeff(gser, z, n), n = 0 .. 40);
# alternative, Jun 18 2011:
g := (2*(1+z^2))/((12*z)*(1+z^2)+sqrt((1+z^2)*(13*z^2))): gser := series(g, z = 0, 45): seq(coeff(gser, z, n), n = 0 .. 40);


CROSSREFS

Cf. A191316.
Sequence in context: A120400 A217283 A000621 * A218020 A318520 A039828
Adjacent sequences: A191314 A191315 A191316 * A191318 A191319 A191320


KEYWORD

nonn


AUTHOR

Emeric Deutsch, Jun 01 2011


STATUS

approved



