For 10 points, identify this namesake of a "phi" function, a prolific Swiss mathematician for whom the number e was supposedly named.
In graph theory, this man lends his name to a closed path that traverses every edge precisely once. The topological formula named for this man states that faces minus edges plus vertices equals two for any polyhedron, giving the surface a value of two for his namesake characteristic. Another equation named for him relates the complex (*) exponential function to cosine of x plus i times sine of x. For 10 points, identify this namesake of a "phi" function, a prolific Swiss mathematician for whom the number e was supposedly named.
In graph theory, this man lends his name to a closed path that traverses every edge precisely once.
Leonhard Euler