Suppose (X,d) is a nonempty complete metric space, f:X→X is a mapping, and denote by fn the nth iterate of f. Suppose for every n there exists a kn>0 such that d(fn(x),fn(y))≤knd(x,y) for all x,y∈X, where ∑n=1∞kn<∞. Prove that f has a unique fixed point in X.