The mathematical constant e is the unique real number such that the function ex has the same value as the slope of the tangent line, for all values of x. More generally, the only functions equal to their own erivatives are of the form Cex, where C is a constant. The function ex so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base e. The number e is also commonly defined as the base of the natural logarithm (using an integral to define the latter), as the limit of a certain sequence, or as the sum of a certain series (see representations of e, below).