Raymond Toy pushed to branch rtoy-issue-78-unneded-code-code-in-complex-acos at cmucl / cmucl

Commits: fe15b0a3 by Raymond Toy at 2020-05-24T08:10:39-07:00 Derive the values on the branch cuts for atanh

Conclusion: atanh is continuous with quadrant Iv for x > 1 and quadrant II for x < -1. This, of course, is consistent with Kahan's coutner-clowkwise continuity principla.

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1 changed file:

- src/code/irrat.lisp

Changes:

===================================== src/code/irrat.lisp ===================================== @@ -1449,7 +1449,44 @@ Z may be any number, but the result is always a complex." ;; +infinity, but the following code returns approx 176 + i*pi/4. The ;; reason for the imaginary part is caused by the fact that arg i*y is ;; never 0 since we have positive and negative zeroes. - +;; +;; The branch cut for atanh is on the real axis for x < -1 and x > 1. +;; Let's derive the values on the branch cut. +;; +;; atanh(z) = 1/2*(log(1+z)-log(1-z)) +;; +;; For z = x, x > 1: +;; atanh(x) = 1/2*(log(1+x) - log(1-x)) +;; = 1/2*(log(1+x) - [log(x-1) + i*arg(1-x)) +;; = 1/2*(log(1+x) - (log(x-1) + i*pi)) +;; = 1/2*(log(1+x) - log(x-1) _ i*pi) +;; = 1/2*log((1+x)/(x-1)) - i*pi/2 +;; +;; For z = -x, x > 1 +;; atanh(x) = 1/2*(log(1-x) - log(1+x)) +;; = 1/2*((log(x-1) + i*arg(1-x)) - log(1+x)) +;; = 1/2*((log(x-1) + i*pi) - log(1+x)) +;; = 1/2*(log((x-1)/(x+1)) + i*pi) +;; = 1/2*log((x-1)/(x+1)) + i*pi/2 +;; +;; For z = x - i0, x > 1 +;; atanh(z) = 1/2*(log(1+x - i0) - log(1-x+i0)) +;; = 1/2*(log(1+x) + i*arg(1+x,-0) - (log(x-1) + i*arg(1-x, +0))) +;; = 1/2*(log(1+x) - i*0 - (log(x-1) + i*pi)) +;; = 1/2*(log(1+x) - log(x-1) - i*pi) +;; = 1/2*log((1+x)/(x-1)) - i*pi/2 +;; +;; This is the same answer we get for atanh(x), x > 1. Hence, atanh +;; is continuous with quadrant IV along the branch cut x > 1. +;; +;; Similary, for z = -x + i0, x > 1: +;; atanh(z) = 1/2*(log(1-x + i0) - log(1+x-i0)) +;; = 1/2*((log(x-1) + i*arg(1-x, +0)) - (log(1+x)+i*arg(1+x, -0))) +;; = 1/2*(log(x-1) + i*pi - (log(1+x) - i0)) +;; = 1/2*(log(x-1) - log(1+x) + i*pi) +;; = 1/2*log((x-1)/(x+1)) + i*pi/2 +;; This is same answer as atanh(x), x < -1. Thus, atanh is continuous +;; with quadrant II on the branch cut for x < -1 (defun complex-atanh (z) "Compute atanh z = (log(1+z) - log(1-z))/2" (declare (number z))

View it on GitLab: https://gitlab.common-lisp.net/cmucl/cmucl/-/commit/fe15b0a31119288135a67ab0...