An Explicit Cubic Iteration

نویسنده

  • P. B. BORWEIN
چکیده

Using the theory of the cubic modular equation we have discovered a remarkably simple class of cubically convergent algebraic iterations for n. In the course of a study of cubic modular equations the following two remarkably simple cubically convergent iterations for n were uncovered. Details of the derivation will appear in [1]. A related but less elegant iteration was discussed in [2]. While derivation of our new algorithm is rather elaborate we begin by describing its genesis. The complete elliptic integrals E and K are defined by t /2 dO. f(/2 K (k) ' = jo ,v/(l_k2sin20), E (k) ' = (1-k2sin20)dO for 0 < k < 1. We write k' := ~//(1-k 2) and K'(k) := K(k'). In these terms the singular value function is defined by solution of gt (k(N)) = x / N for positive N. This uniquely defines k on [0, ~) as a decreasing function with k(0) = m, k (1) = 1/,,/2, k (m) = 0. It is known that k is algebraic when N is rational [5]. Various related invariants aretabulated below. Moreover, for some N one or more of these invariants become very simple. In [1] a corresponding function a (a singular value of the second kind) was studied. It is defined by E' n (1) a(N)-K 4K 2 k "= k(N) and also is algebraic at rational values. It is antitone on [1, ~) with ~(1) = I/2 and ~ (~) = i/n. Moreover, it satisfies many recursions which allow one to com

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تاریخ انتشار 2005