|
|
A186031
|
|
Number of Dyck paths of semilength n with a valley (DU) spanning the midpoint.
|
|
1
|
|
|
0, 1, 1, 5, 10, 42, 107, 429, 1234, 4862, 15032, 58786, 190588, 742900, 2490399, 9694845, 33312770, 129644790, 453999656, 1767263190, 6282014804, 24466267020, 88026769844, 343059613650, 1246635155180, 4861946401452, 17815452662152
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,4
|
|
LINKS
|
|
|
FORMULA
|
|
|
EXAMPLE
|
For n=4 the 5 paths are UDUD.UDUD, UUDD.UDUD, UDUD.UUDD, UUDD.UUDD and UUUD.UDDD where dot marks the midpoint.
|
|
MAPLE
|
C:= n-> binomial (2*n, n)/(n+1):
a:= n-> C(n-1) -`if` (irem(n, 2)=1, C((n-1)/2)^2, 0):
seq (a(n), n=1..40);
|
|
CROSSREFS
|
Dyck n-paths with peak at midpoint is given by A000108(n-1); For UU (or DD) at midpoint see A027302.
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|