With Lagrange, this man n
With Lagrange, this man names a differential equation whose solutions are stationary for some functional; that equation appears in the calculus of variations. He is alphabetically the first of two co-namers of a constant that measures the difference between the harmonic series and the natural log. He names the first-order Runge-Kutta method, the simplest method for integrating ODEs. One quantity named for him is an invariant that equals 2 for all convex polyhedra. He devised his namesake cycles while studying the bridges of Königsberg. His identity states that "e to the i pi equals negative one". For 10 points, name this Swiss mathematician for whom the constant e is named.
With Lagrange, this man names a differential equation whose solutions are stationary for some functional; that equation appears in the calculus of variations.
Leonhard Euler [pronounced "oiler", but accept any reasonable pronunciation]