In addition to being a clergyman my entire life, I studied mathematics under Bonaventura Cavalieri at the University of Bologna and succeeded him as a professor in 1647, remaining there for the next 39 years.
My most important mathematical work explored the convergence of series and integration, such as proving that for a series to converge, its terms also must converege to zero.
Preceding the work of Jacob Bernoulli, I showed that the harmonic series diverges....and in 1650, proved that the sum of the alternating harmonic series is equal to ln(2).
Some other math I played with included posing in 1644 the famous "Basel problem," which asks for the sum of the reciprocals of the squares of the natural numbers...it was eventually solved in 1735 by Leonhard Euler at age 28.
Finally, I was the first to prove that Wallis' product of terms converged to π.
Pietro Mengoli (1626 - 1682)