Suppose the function \(f\) satisfies the equation \(f(x+y) = f(x)f(y)\) for all \(x\) and \(y\). Here \(f(x) = 1 + xg(x)\), where \(\displaystyle\lim_{x \to 0} g(x) = T\), and \(T\) is a positive integer. If \(f^{n}(x) = kf(x)\), where \(f^{n}(x)\) denotes the \(n\)th derivative of \(f\), then \(k\) is equal to: