Formula which calculates the normal curvature of an arbitrary direction in the tangent plane in terms of the principal curvatures \kappa_1 and \kappa_2 and the angle \theta which that direction makes with the first principal direction:
Either of two important mathematical theorems of Leonhard Euler. The first is a topological invariance (see topology) relating the number of faces, vertices, and edges of any polyhedron. It is written F + V = E + 2, where F is the number of faces, V the number of vertices, and E the number of edges. A cube, for example, has 6 faces, 8 vertices, and 12 edges, and satisfies this formula. The second formula, used in trigonometry, says e^ix = cos x + isin x where e is the base of the natural logarithm and i is the square root of -1 (see irrational number). When x is equal to or 2, the formula yields two elegant expressions relating , e, and i: e^i = -1 and e^2i = 1
born April 15, 1707, Basel, Switz. died Sept. 18, 1783, St. Petersburg, Russia Swiss mathematician. In 1733 he succeeded Daniel Bernoulli (see Bernoulli family) at the St. Petersburg Academy of Sciences. There he developed the theory of trigonometric and logarithmic functions and advanced mathematics generally. Under the patronage of Frederick the Great, he worked at the Berlin Academy for many years (1744-66), where he developed the concept of function in mathematical analysis and discovered the imaginary logarithms of negative numbers. Throughout his life he was interested in number theory. In addition to inspiring the use of arithmetic terms in writing mathematics and physics, Euler introduced many symbols that became standard, including for summation; n for the sum of divisors of n; e for the base of the natural logarithm; a, b, and c for the sides of a triangle with A, B, and C for the opposite angles; f(x) for a function; for the ratio of the circumference to the diameter of a circle; and i for SquareRoot(-1). He is considered one of the greatest mathematicians of all time