Euler's identity (or ``theorem'' or ``formula'') is
To ``prove'' this, we will first define what we mean by `` ''. (The right-hand side, , is assumed to be understood.) Since is just a particular real number, we only really have to explain what we mean by imaginary exponents. (We'll also see where comes from in the process.) Imaginary exponents will be obtained as a generalization of real exponents. Therefore, our first task is to define exactly what we mean by , where is any real number, and is any positive real number.