Complex sentence: Importantly, the core of their insight was the formalization of the inverse properties between the integral and the Barrow who had created formulas for specific cases and Fermat who created a similar definition for the derivative.
Split into simpler sentences: Importantly, the core of their insight was the formalization of the inverse properties between the integral and the differential. Foremost among these was Barrow who had created formulas for specific cases and Fermat who created a similar definition for the derivative.