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Leonhard Euler was one of the most prolific mathematicians in history, and also a prolific inventor of canonical notation. His contributions include his use of "e" to represent the base of natural logarithms. It is not known exactly why formula_6 was chosen, but it was probably because the four letters of the alphabet were already commonly used to represent variables and other constants. Euler used formula_5 to represent pi consistently. The use of formula_5 was suggested by William Jones, who used it as shorthand for perimeter. Euler used formula_13 to represent the square root of negative one, although he earlier used it as an "infinite number." For summation, Euler used sigma, Σ. For functions, Euler used the notation formula_14 to represent a function of formula_15. In 1730, Euler wrote the gamma function. In 1736, Euler produces his paper on the Seven Bridges of Königsberg initiating the study of graph theory.