Branched Complex Functions (2)

We begin by solving the problem from the last instalment:

Problem: Show that Log z is well defined and single-valued when z not equal 0
(Hint: use z = r exp(i0)

Solution:  z = r exp(i0) = r exp(0) = r(1) = r

Then: Log z = ln r

so Log r = ln r

Now, a branch of a multiple-valued function f is any single-valued function F that is analytic at some domain at a point z for which the value F(z) = f(z). (Being “analytic” means only that F cannot take on random values of f), Then for each fixed real number, k, the single-valued function

e.g. log z= ln r + i(q)   (r > 0, k < q < k + 2 p)

is a branch of the multiple –valued function:

log z= ln r + i(q) 

Then the function:

Log z = ln r + iQ   (r > 0, - p < Q < p)

Is called the principal branch.

Now, a branch cut is a portion of a line or curve that is introduced in order to define a branch F of a multiple –valued function f. Points on the branch cut for F are singular points of F, and any point that is common to all branch cuts of f is called a branch point.

Example: For a sunspot model


In the diagram shown, a multipole sunspot model is depicted such that the most intense magnetic field locus is centered in the largest sunspot and is "line-tied" , i.e. it represents the anchored ‘foot’ of one magnetic arch with the other foot connected to a 2nd polarity (minor spot) executing proper motion around it.

In this case the dipole undergoes an approximate 28 degree proper motion over time interval (t2  - t1) during which the (-) polarity footpoint is displaced from a1 to a2.  This translates the location of the field line f1 from roughly a1 = (-3k – 3ki) where each k =  10 ⁴ km, to  a2  (-k – 4ki). The neutral line or magnetic inversion line itself passes through the sunspot along a defined Re x -axis.   Here, we designate the ray Q = 2 p, as a branch cut along the positive Re-axis. This is really the branch cut for the functions f and F to be worked out. The origin (x=0, y=0) is putatively the branch point for the system.

In effect, the branch cut is a barrier separating the domain of one branch of a function from a different branch. A multiple –valued function, meanwhile, is discontinuous at every point of a branch cut for the function.  Setting the origin for our sunspot system evidently means setting branch points for branches of a multiple-valued logarithmic function too.

Problems for the Math Maven:

1) Using the diagram for the multipole sunspot shown, identify the principal branch for the given branch cut. (Hint: express as a function of r, Q)

2) Consider the complex z-plane, i.e. z(x, iy) featuring concentric circles of differing radii a = 0.5, 1, and 2 such that the equation:  êz ê   =  r = a  applies. Show these concentric circles and then show at least three lines on the same coordinate system which satisfy q =  a . (where a  is any designated angle)


3) For the multipole sunspot diagram shown, assume the ray Q1(a1,t1) = 6 p/5 and Q2(a2,t2) = 7 p/5 for a point z defined in proper motion about the origin, anchored in the major spot. Can you write a function describing this proper motion? (Hint: w = u + iv = ln z = ln r + iq, , so that u = ln r and v = q, .). Be sure to cite any assumptions made.