A value named for this man is proportional to the 
A value named for this man is proportional to the integral of Gaussian curvature over a closed surface. He is the namesake of an identity stating that "cosine theta plus sine theta times i" equals "e to the i times theta." His namesake "characteristic" equals vertices minus edges plus faces. The limit of the difference between the harmonic series and the natural log is named for Mascheroni and this solver of the Bridges of Konigsberg problem. A value of about 2.718, the base of the natural log, is named e in his honor. For 10 points, name this Swiss mathematician.
A value named for this man is proportional to the integral of Gaussian curvature over a closed surface.
Leonhard Euler