The document discusses recurrences and methods for solving them. It covers: 1) Divide-and-conquer algorithms can often be modeled with recurrences. Examples include merge-sort and matrix multiplication. 2) Common methods for solving recurrences are substitution, iteration/recursion trees, and the master method. The master method provides a general solution for recurrences of the form T(n) = aT(n/b) + nc. 3) Strassen's matrix multiplication algorithm improves on the naive O(n^3) time by using a recurrence with a=7 to achieve O(n^2.81) time via the master method. Changing variables can sometimes simplify recurrences.