For now, K x is defined by this result (its significance will be explained shortly). In the limit of an infinite number of infinitely small steps, the finite boost transformation in the form of a matrix exponential is obtained B x = lim N → → ∞ ∞ ( I − − ζ ζ N K x ) N = e − − ζ ζ K x where the limit definition of the exponential has been used (see also characterizations of the exponential function). More generally