Today marks the 306th birthday of famous Swiss mathematician, Leonhard Euler (1707-1783), as you might have noticed from the unique Google logo. Euler made a significant number of discoveries in various fields, from infinitesimal calculus to graph theory. He is well known for introducing the notion of a mathematical function and his works take up between 60-80 quarto volumes. Here are just some of his accomplishments.

## Euler’s formula

Euler’s formula states that if a finite, connected, planar graph is drawn in the plane without any edge intersections, and v is the number of vertices, e is the number of edges and f is the number of faces (regions bounded by edges, including the outer, infinitely large region), then

v − e + f = 2.

## Seven Bridges of Königsberg

The Seven Bridges of Königsberg is a historically notable problem in mathematics. Its negative resolution by Leonhard Euler in 1735 laid the foundations of graph theory and prefigured the idea of topology.

The problem was to find a walk through the city that would cross each bridge once and only once. The islands could not be reached by any route other than the bridges, and every bridge must have been crossed completely every time; one could not walk halfway onto the bridge and then turn around and later cross the other half from the other side. The walk need not start and end at the same spot. Euler proved that the problem has no solution. There could be no non-retracing the bridges. The difficulty was the development of a technique of analysis and of subsequent tests that established this assertion with mathematical rigor.

## Euler’s identity

In analytical mathematics, Euler’s identity (also known as Euler’s equation), named for the Swiss mathematician Leonhard Euler, is the equality

where

e is Euler’s number, the base of natural logarithms,

i is the imaginary unit, which satisfies i2 = −1, and

π is pi, the ratio of the circumference of a circle to its diameter.

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