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Leonhard Euler(1707 - 1783) was a Swiss mathematician who is famous for much work in mathematics in particular for his studies of polyhedra, about which he discovered a rule:
The number of Vertices + the number of Faces - the number of Edges = 2
This formula also applies to networks: the number of Nodes + the number of Regions - the number of Arcs = 2

Leonhard Euler (1707-83):
A Swiss mathematician who worked on analysis, calculus, planetary motion, mechanics, etc. He invented a lot of the notation we use today, eg the sigma notation for sums. He discovered a formula relating the number of vertices, edges and faces of a polyhedron; he solved the Königsberg bridge problem and invented the φ function for the number of integers smaller than and coprime to n.