Department of Mathematics

Colorado State University

Math 301: A new counting problem

Class date: Monday, September 23, 2019
  1. Recall our “staircase walk” problem from the last classwork. Let \(n \in \mathbb N\). We are working on a grid, where we are allowed only to take unit-length UP steps and unit-length RIGHT steps.

    1. How many ways are there to get from the point \((0,0)\) to the point \((n,n)\) using our allowed steps?

      This question is equivalent to, “How many words from the alphabet {U,R} of length \(2n\) have \(n\) U’s and \(n\) R’s?” The answer is thus \(\binom {2n}{n}\).

    2. How many ways are there to get from the point \((0,0)\) to the point \((n-1, n+1)\) using our allowed steps?

      Similar to part a, this question is equivalent to, “How many words from the alphabet {U,R} of length \(2n\) have \(n+1\) U’s and \(n-1\) R’s?” The answer is thus \(\binom {2n}{n-1} = \binom{2n}{n+1}\).

    3. Find a bijection between walks from \((0,0)\) to \((n,n)\) that go above the line \(x = y\) and all walks from \((0,0)\) to \((n-1, n+1)\).

      Hint: If a walk goes about \(x = y\), then there is a first point where it lands on the line \(y = x+1\).

      Notice that if a path from \((0,0)\) to \((n,n)\) ever crosses above the line \(y = x\), it touches the line \(y = x+1\). Since it touches the line, there is a part of the path that is the first place it touches the line. The same can be said for any path from \((0,0)\) to \((n-1,n+1)\); since the start point is below the line but the end point is above the line, any path that gets to \((n-1,n+1)\) has to touch \(y = x+1\)!

      Take any path from \((0,0)\) to a point \(P\) that touches the line \(y = x+1\). The path touches the line at some first point, \(Q\). Fix the path from \((0,0)\) to \(Q\) constant, but reflect the path from \(Q\) to \(P\) through the line \(y = x+1\). This reflection interchanges up steps and right steps, so the path is still one satisfying the rules laid out.

      Notice that the reflection of \((n,n)\) is \((n-1, n+1)\) and vice versa. This reflection procedure thus describes a bijective correspondence between walks from \((0,0)\) to \((n,n)\) that go above the line \(x = y\) and all walks from \((0,0)\) to \((n-1, n+1)\).

      We conclude that the number of walks from \((0,0)\) to \((n,n)\) that go above \(y=x\) is \(\binom {2n}{n-1}\).

    4. Find a formula for the number of walks from \((0,0)\) to \((n,n)\) that do not go above the line \(x = y\).

      If we have a walk that goes from \((0,0)\) to \((n,n)\) which was not counted in part c, it must be a walk that we want to count here. So, the count of such walks is;

      \[ \binom{2n}{n} - \binom{2n}{n-1} = \frac{1}{n+1}\binom{2n}{n}. \]

      This number is also called the \(n\)th Catalan number \(C_n\), and it is the answer to many different counting problems, as we’ll see in question 2.

  2. Consider words of length \(2n\) from the alphabet \(\{ (,) \}\).

    1. How many such words are there?

      There are 2 letters in the alphabet and \(2n\) letters per word, so \(2^{2n}\).

    2. How many such words are there with equal numbers of open- and closed- parentheses?

      This is a question we’ve answered before. There are \(2n\) letters to be chosen, and \(n\) (that is, half) of them must be \((\). So we pick the \(n\) spots and the rest must be filled with \()\). So, there are \(\binom {2n}{n}\) such words.

    3. How many of these words are valid parenthesizations? For example \((())()\) is valid, since each open parenthesis has a paired closed parenthesis to its right, while \()()(()\) is invalid, since it has an open parenthesis with no closing parenthesis to its right.

      Hint: Find a bijection with paths counted in 1.d.

      If we change the letter \((\) to a right step and \()\) to an up step, notice that valid parenthesizations are exactly words that pair with paths counted in 1.d that do not go above the line \(y = x\). So, the number of valid parenthesizations is \(C_n\), the \(n\)th Catalan number:

      \[ \frac{1}{n+1}\binom{2n}{n} \]