Complex sentence: The recursion is repeated until the zero-containing rectangles are either small enough that their centres give sufficiently accurate zero estimates or, alternatively, that another root-finding algorithm can be applied to the estimates to further refine them.
Split into simpler sentences: The recursion is repeated until the zero-containing rectangles are either small enough that their centres give sufficiently accurate zero estimates. Alternatively, another root-finding algorithm can be applied to the estimates to further refine them.