The cubic formula requires taking cube roots of the two solutions of a quadratic equation. When you try to use it to find the real (or imaginary) part of cbrt(a+bi), you wind up facing the same problem (or an equivalent) over again. Numerically, your options for cbrt(a+bi) are trigonometric (choice of sin/cos/tan), logarithmic & exponentials of complex numbers (coming back to trig), or Newton's method (& siblings), or the HP-calculator method, or a power series solution based on reverting the series for x-x^3 (it's cute). It looks like the schoolbook method for cbrt could be adapted to work, by rotating a+bi so that Realpart>=|Imagpart|, and going alternately after a bit/digit of the real & imaginary roots. Rich PS: Suppose you have a subroutine for cbrt(a+bi) on the unit circle, but nothing for cbrt(real). Can you somehow get cbrt(2)? --R -----Original Message----- From: math-fun-bounces@mailman.xmission.com [mailto:math-fun-bounces@mailman.xmission.com] On Behalf Of victor miller Sent: Wednesday, November 11, 2009 9:57 PM To: Dan Asimov; math-fun Subject: Re: [math-fun] Cube root of a complex number Another way of seeing this is (despite your admonishment not to mention trig functions) is that we really only have to do it when |a+bi|=1. Using the fact that cos(3 theta) = 4 cos^3 theta - 3 cos theta, so we see that we need to solve the cubic: 4 t^3 - 3 t - a = 0. This is the casus irreducibilis of solving the cubic. Even though all the roots are real, when using radicals, one needs to take the cube root of a non-real quantity: http://en.wikipedia.org/wiki/Casus_irreducibilis Victor On Wed, Nov 11, 2009 at 10:38 PM, Dan Asimov <dasimov@earthlink.net> wrote:
No, that's perfect, since the equations for x and y can be solved by the solution to the general cubic . . . and will *hopefully* give real answers -- leading to expressions for the real and imaginary parts of (a + bi)^(1/3) in terms of radicals, which is what I was looking for.
--Dan
James wrote:
<< Oops, I seem to have x and y reversed below.
On Wed, Nov 11, 2009 at 9:01 PM, James Buddenhagen <jbuddenh@gmail.com> wrote: Not as pretty, and not really a formula, but sort of interesting:
Let the cube root of a+bi be x^(1/3) + y^(1/3) i. Then x is a root of 64x^3 - 48bx^2 - (27a^2+15b^2)x - b^3 = 0, and y is a root of 64y^3 + 48ay^2 - (27b^2+15a^2)y + a^3 = 0.
On Wed, Nov 11, 2009 at 7:13 PM, Dan Asimov <dasimov@earthlink.net> wrote: << It's a pleasant exercise to calculate the square root of the complex number a + bi directly (without making use of polar form or trig functions).
(Be sure to consider all cases.)
But is there a similar formula for the cube root of a + bi ???
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