We consider the complexity for computing the approximate sum $a_1+a_2+...+a_n$ of a sorted list of numbers $a_1\le a_2\le ...\le a_n$. We show an algorithm that computes an $(1+\epsilon)$-approximation for the sum of a sorted list of nonnegative numbers in an $O({ 1\over \epsilon}\min(\log n, { \log ({ x_{ max}\over x_{ min}})})\cdot (\log { 1\over \epsilon}+\log\log n))$ time, where $x_{ max}$ and $x_{ min}$ are the largest and the least positive elements of the input list, respectively. We prove a lower bound $\Omega(\min(\log n,\log ({ x_{ max}\over x_{ min}}))$ time for every O(1)-approximation algorithm for the sum of a sorted list of nonnegative elements. We also show that there is no sublinear time approximation algorithm for the sum of a sorted list that contains at least one negative number.
Categories: cs.DS cs.CC