I've been reading this paper on the persistence algorithm:
Given a matrix M with columns $M_j$.
it states on page 3 that we may add two columns $M_j+ M_i$ and store it at $M_j$ in time $\#M_i * log(\#M_i + \#M_j)$
where $\#M_i$ is the number of nonzero entries of column j.
using a binary search tree to represent the nonzero entries of each column.
Can anyone explain how the column addition would work? I think I'm just missing something very simple.