Catalan Numbers

Suppose you have n pairs of parentheses and you would like to form valid groupings of them, where “valid” means that each open parenthesis has a matching closed parenthesis. For example, “(()())” is valid, but “())()(” is not. How many groupings are there for each value of n? Perhaps a more precise definition of the problem would be this: A string of parentheses is valid if there are an equal number of open and closed parentheses and if you begin at the left as you move to the right, add 1 each time you pass an open and subtract 1 each time you pass a closed parenthesis, then the sum is always non-negative. Table 1 shows the possible groupings for 0 ≤ n ≤ 5.