- #1
Bashyboy
- 1,421
- 5
Homework Statement
Determine the points for which ##f(z) = \frac{z^2 + 3}{(z-3)^2(z^2 - 4z + 5)}## is analytic and singular.
Homework Equations
Theorem 133:
Suppose c is a complex constant and suppose the derivatives of the complex
functions f and g exist at z. Then
1. Sums
##\frac{d}{dz} \left[f(z) + g(z) \right] = f'(z) + g'(z)##
2. Product rule
##\frac{d}{dz} \left[f(z)g(z) \right] = f'(z) g(z) + f(z) g'(z)##
3. Quotient rule. For those values of ##z## where ##g(z) \ne 0##,
##\frac{d}{dz} \left[\frac{f(z)}{g(z)} \right] = \frac{f(z) g'(z) - f'(z) g(z)}{[g(z)]^2}##
The Attempt at a Solution
Because the question of analyticity is related to a functions differentiability, before we determine at which points it is analytic and singular, let us first ascertain its differentiability.
The function ##f(z)## is a ratio of the two functions ##g(z) = z^2 + 3## and ##h(z) = (z-3)^2(z^2 - 4z + 5)##. Theorem 133, part 3, states that, if the functions ##f(z)## and ##g(z)## are differentiable at the point ##z##, then their quotient ##\frac{f(z)}{g(z)}## is differentiable at the same point ##z##, provided that ##g(z)## is not zero at this point; if it is, then the quotient is not differentiable at that point. We are therefore urged to find at the points for which ##h(z)## is zero:
##
(z-3)^2(z^2 - 4z + 5) = 0 \implies
##
##
(z - 3)^2 = 0 \implies z = 3
##
and
##
(z^2 - 4z + 5) = 0 \implies (z-5)(z+1) = 0 \implies z=-1 ~~~\mbox{or}~~~ z = 5
##
Therefore, the rational function ##f(z)## will not be differentiable at these three points.
Are there any other points at which ##h(z)## is not differentiable? From the same theorem alluded to above, the product rule tells us a similar thing which the quotient rule tells us. The function that is a product of two function is differentiable at every point that the functions in the product are. Sure enough, from problem 130 we know that ##z## and ##z^2## are differentiable functions; and from theorem 131 we know that a function which is the sum of two differentiable functions is differentiable everywhere the two functions are. For instance, because we know that ##z## is differentiable everywhere, as well as any constant function, we know that ##z-3## is also a differentiable function. Therefore, ##h(z)## is differentiable everywhere.
How about the function ##g(z)##; at which points is it differentiable? The same observations will reveal that ##g(z) = z^2 + 3## is differentiable everywhere. Therefore, the function is not differentiable at the points ##\{-1,3,5\}##, meaning that they are points of singularity. The function is analytic everywhere else.
Pardon me if this sounds rather dull and too formal (this is for homework I have to hand). I just curious to know if my reasoning is sound. I would appreciate it if someone could check my logic.