Revisiting Gilbert Strang's "a chaotic search for i"

Abstract

In the paper "A Chaotic Search for i" ([25]), Strang completely explained the behaviour of Newton's method when using real initial guesses on f(x) = x² + 1, which has only the pair of complex roots ±i. He explored an exact symbolic formula for the iteration, namely x_n = cot (2ⁿθ₀), which is valid in exact arithmetic. In this paper, we extend this to to k^th order Householder methods, which include Halley's method, and to the secant method. Two formulae, x_n = cot (θ_n-1 + θ_n-2) with θ_n-1 = arccot (x_n-1) and θ_n-2 = arccot (x_n-2), and x_n = cot ((k + 1)ⁿθ₀) with θ₀ = arccot(x₀), are provided. The asymptotic behaviour and periodic character are illustrated by experimental computation. We show that other methods (Schröder iterations of the first kind) are generally not so simple. We also explain an old method that can be used to allow Maple's Fractals[Newton] package to visualize general one-step iterations by disguising them as Newton iterations.