The most influential mathematician of the 18th century was arguably可以说 Leonhard Euler. His contributions range from founding the study of graph theory图论 with the Seven Bridges of Königsberg problem七桥问题 to standardizing many modern mathematical terms术语 and notations符号. For example, he named the square root of minus 1 with the symbol i, and he popularized推广 the use of the Greek letter to stand for the ratio of a circle's circumference圆周率 to its diameter. He made numerous contributions to the study of topology拓扑学, graph theory, calculus, combinatorics, and complex analysis, as evidenced by the multitude众多、大量 of theorems and notations named for him. Other important European mathematicians of the 18th century included Joseph Louis Lagrange, who did pioneering work in number theory数论, algebra, differential calculus微分学, and the calculus of variations变分学, and Laplace who, in the age of Napoleon, did important work on the foundations of celestial mechanics天体力学 and on statistics统计学.