Exercise 7.4.1

Show that in the recurrence

$$ T(n) = \max_{0 \le q \le n-1} (T(q) + T(n-q-1)) + \Theta(n) $$

$$ T(n) = \Omega(n^2) $$

We guess $T(n) \ge cn^2 - 2n$:

$$ \begin{align} T(n) &= \max_{0 \le q \le n-1} (T(q) + T(n-q-1)) + \Theta(n) \\ &\ge \max_{0 \le q \le n-1} (cq^2 - 2q + c(n-q-1)^2 - 2n - 2q -1) + \Theta(n) \\ &\ge c\max_{0 \le q \le n-1} (q^2 + (n-q-1)^2 - (2n + 4q + 1)/c) + \Theta(n) \\ &\ge cn^2 - c(2n-1) + \Theta(n) \\ &\ge cn^2 - 2cn + 2c & (c \le 1) \\ &\ge cn^2 - 2n \end{align} $$