Skip to content# Catalan Numbers

**Classifications**

Author(s): Tom Davis

Suppose you have n pairs of parentheses and you would like to form valid groupings of them, how many groupings are there for each value of n? How many “mountain ranges” can you form with n upstrokes and n downstrokes that all stay above the original line? What about counting the number of ways to triangulate a regular polygon with n + 2 sides? We begin with a set of problems that will be shown to be completely equivalent. The solution to each problem is the same sequence of numbers called the Catalan numbers.

Topics covered: Combinatorics

Session styles: Multiple Representations, Try a Smaller Problem

Mathematical practices: MP1 – Make sense of problems and persevere in solving them., MP2 – Reason abstractly and quantitatively., MP3 – Construct viable arguments and critique others’ reasoning., MP6 – Attend to precision., MP7 – Look for and make use of structure., MP8 – Look for and express regularity in repeated reasoning.

Supporting materials: Facilitator Guides

Associated Math Circles: MTC Network