# Exercise 15.2.4

Describe the subproblem graph for matrix-chain multiplication with an input chain of length $n$. How many vertices does it have? How many edges does it have, and which edges are they?

Assuming this is the efficient algorithm, the graph has vertices $v_{ij}$ where $i \le j$. There are $1 + 2 + \ldots + n$ of them, which is $\frac{n(n+1)}{2}$, a boring old formula.

As for edges, each vertex has two edges, $(v_{ij}, v_{ik})$ and $(v_{ij}, v_{k+1,j})$ for each $k$ such that $i \le k < j$. The sum is:

$$ \sum_{i=1}^n \sum_{j=i}^n 2 (j - i) $$

This is probably expandable, but I'm not that hard-working.