Exercise III: Catalan numbers For n e N, denote by c, to be the number of ways to form a "mountain range" with n upstrokes (U) and n downstrokes (D) that all stay above a horizontal line. For instance: • for n = 1, only UD is allowed, so c1 = 1; • for n = 2, only UUDD and U DUD are allowed, so c2 = 2. 1. Check that c3 = 5 by writing down or drawing all possible options. 2. Consider the power series „a" 3 = (x)6 n=0 (by definition co = 1) called the generating function of the sequence (en). Justify that Cn < 4" for each n, and deduce that the radius of convergence of g is at least 1/4.

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Exercise III: Catalan numbers
For n e N, denote by c, to be the number of ways to form a "mountain range" with n
upstrokes (U) and n downstrokes (D) that all stay above a horizontal line. For instance:
for n = 1, only UD is allowed, so c = 1;
• for n = 2, only UUDD and UDUD are allowed, so c2 = 2.
1. Check that c3 = 5 by writing down or drawing all possible options.
2. Consider the power series
+00
g(x) = 2
n=0
(by definition co =
1) called the generating function of the sequence (en). Justify
that cn < 4" for each n, and deduce that the radius of convergence of g is at least
1/4.
3. It can be show that for r E (-1/4, 1/4),
g(x) = 1+ xg(r)?
and therefore
1- VI
4.x
g(x) =
2.x
Use this formula and the known power series of V1+ x to write the first terms of
the power series expansion of g, in the form
g(x) = co + c1x + c2x2 + C3x + c4x* + ·..
Show how you obtain a few terms, but you do not need to show all computations
and you can use a calculator for fractions. Check that you recover co, C1, C2, C3, and
deduce the value of c4.
4. (Bonus, ungraded, for fun) Check your result by writing down or drawing all possible
paths for n = 4.
Transcribed Image Text:Exercise III: Catalan numbers For n e N, denote by c, to be the number of ways to form a "mountain range" with n upstrokes (U) and n downstrokes (D) that all stay above a horizontal line. For instance: for n = 1, only UD is allowed, so c = 1; • for n = 2, only UUDD and UDUD are allowed, so c2 = 2. 1. Check that c3 = 5 by writing down or drawing all possible options. 2. Consider the power series +00 g(x) = 2 n=0 (by definition co = 1) called the generating function of the sequence (en). Justify that cn < 4" for each n, and deduce that the radius of convergence of g is at least 1/4. 3. It can be show that for r E (-1/4, 1/4), g(x) = 1+ xg(r)? and therefore 1- VI 4.x g(x) = 2.x Use this formula and the known power series of V1+ x to write the first terms of the power series expansion of g, in the form g(x) = co + c1x + c2x2 + C3x + c4x* + ·.. Show how you obtain a few terms, but you do not need to show all computations and you can use a calculator for fractions. Check that you recover co, C1, C2, C3, and deduce the value of c4. 4. (Bonus, ungraded, for fun) Check your result by writing down or drawing all possible paths for n = 4.
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