**Leonhard Euler**(/

**OY***-lər)*15 April 1707– 18 September 1783) was a pioneering Swiss mathematician and physicist. He made important discoveries in many fields. He also introduced much of the modern mathematical terminology and notation. He is also renowned for his work in mechanics, fluid dynamics, optics, astronomy, and music theory.

Euler is considered to be the most famous
mathematician of the 18th century and one of the greatest
mathematicians to have ever lived. He is also one of the most
prolific mathematicians; his books fill 60–80 volumes. He spent
most of his adult life in St. Petersburg, Russia and Berlin, Prussia.

**Seven Bridges of Königsberg**is a historically notable problem in mathematics. Leonhard solved this problem in 1735.

The
problem was to find a way to walk through the city that would cross each of 7 bridges 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 does not need to start and end at the same spot).
Euler didn't show a way to walk the seven bridges, but he proved that the problem has

**no**solution- there was no way to do it.
To solve this problem,
Euler realized that the shape of the island and city do not matter.
The only thing that matters is the number of bridges and where they
connect, like so:

In modern mathematical terms, each land mass becomes a "vertex" (blue dot), and each bridge
becomes an "edge” (line), the resulting mathematical figure is called
a “graph”.

If you can draw a line that starts on a vertex and covers each
edge once and only once without lifting the pencil, you have solved
the problem by proving it is possible. Remember that Euler proved that this graph is impossible, which means you cannot draw the graph without lifting the pencil and starting at a new vertex.

Can you guess how Euler knew? Look for a new blog showing you how to tell whether you can draw a figure without lifting the pencil.

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