Difference between revisions of "Catalan sequence"

Revision as of 19:34, 8 December 2007

The Catalan numbers are a sequence of positive integers that arise as the solution to a wide variety of combinatorial problems. The first few Catalan numbers are $C_0 = 1$ , $C_1 = 1$ , $C_2 = 2$ , $C_3 = 5$ , .... In general, the $n$ th Catalan number is given by the formula $C_n = \frac{1}{n + 1}\binom{2n}{n}$ , where $\binom{2n}{n}$ is the $n$ th central binomial coefficient.

Introduction

Catalan numbers can be used to find:

The number of ways to arrange $n$ pairs of matching parentheses.
The number of ways a convex polygon of $n+2$ sides can be split into $n$ triangles by $n - 1$ nonintersection diagonals.
The number of rooted binary trees with exactly $n+1$ leaves.

Example

In how many ways can the product of $n$ ordered number be calculated by pairs? For example, the possible ways for $a\cdot b\cdot c\cdot d$ are $a((bc)d), a(b(cd)), (ab)(cd), ((ab)c)d,$ and $(a(bc))d$ .

Solution

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