In 1736, he proved that no (*) path crossed each of the Pregel River's seven bridges in Konigsberg exactly once.
This man found the sum of the squares of the reciprocals of all natural numbers, thus solving the Basel problem, to be pi²/6 ["pi squared over six"]. The limiting difference between the natural log function and the harmonic series is a constant named for him and Mascheroni [mas-kair-OH-nee]. In 1736, he proved that no (*) path crossed each of the Pregel River's seven bridges in Konigsberg exactly once. Graph theory was essentially invented by, for 10 points, what Swiss mathematician and namesake of a constant about equal to 2.718, called e?
This man found the sum of the squares of the reciprocals of all natural numbers, thus solving the Basel problem, to be pi²/6 ["pi squared over six"].
Leonhard (Paul) Euler [OY-lur]