how many shortest paths from a to b do not pass …jjlee/teaching/intro_to_catalan.pdf · how many...
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How many shortest paths from A to B do not pass abovethe diagonal?
A
B
Discrete Mathematics Catalan Numbers
A
B
Blue path is good
Discrete Mathematics Catalan Numbers
A
B
Blue path is good
Discrete Mathematics Catalan Numbers
A
B
Red path is bad
Discrete Mathematics Catalan Numbers
A
B
Red path is bad
Discrete Mathematics Catalan Numbers
Number of good paths
= Total Number of paths− Number of bad paths
=(2n)!n!n!
− Number of bad paths
So it is sufficient to count the number of bad paths.
Discrete Mathematics Catalan Numbers
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− Number of bad paths
So it is sufficient to count the number of bad paths.
Discrete Mathematics Catalan Numbers
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− Number of bad paths
So it is sufficient to count the number of bad paths.
Discrete Mathematics Catalan Numbers
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− Number of bad paths
So it is sufficient to count the number of bad paths.
Discrete Mathematics Catalan Numbers
How to count the number of bad paths?
A
B
Discrete Mathematics Catalan Numbers
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
A
B
Discrete Mathematics Catalan Numbers
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
A
B
(n, n+ 1)
A
B
(n, n+ 1)
In fact, reflection turns every bad path into a path reaching (n−1, n+1).
Discrete Mathematics Catalan Numbers
Moreover, every path reaching (n− 1, n + 1) is obtained from a bad path.
A
B
(n, n+ 1)
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
Moreover, every path reaching (n− 1, n + 1) is obtained from a bad path.
A
B
(n, n+ 1)
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
Moreover, every path reaching (n− 1, n + 1) is obtained from a bad path.
A
B
(n, n+ 1)
A
B
(n, n+ 1)
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1)
, which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths
= Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)
The number Cn = 1n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers
Conclusion: There is a one-to-one correspondence between the set of badpaths and the set of paths reaching (n− 1, n + 1).
Therefore the number of bad paths must equal the number of paths
reaching (n− 1, n + 1), which is (2n)!(n−1)!(n+1)! .
Finally,
Number of good paths = Total Number of paths− Number of bad paths
=(2n)!n!n!
− (2n)!(n− 1)!(n + 1)!
=1
n + 1
(2n
n
)The number Cn = 1
n+1
(2nn
)is called the nth Catalan number and has a
lot of applications.
Discrete Mathematics Catalan Numbers