• Title: Wirtinger Derivatives

  • Series: Complex Analysis

  • YouTube-Title: Complex Analysis 8 | Wirtinger Derivatives

  • Bright video: https://youtu.be/5tJ-EDx7MlM

  • Dark video: https://youtu.be/l1yJmTCoylM

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  • Subtitle on GitHub: ca08_sub_eng.srt

  • Timestamps

    00:00 Intro

    00:19 Wirtinger derivatives ā€” basic definition

    01:35 Complex derivative through Cauchy-Riemann equations

    05:12 Wirtinger derivatives ā€” detailed definition

    06:17 Example for zĀ²

    08:39 Summary: a criteria for holomorphic functions

  • Subtitle in English

    1 00:00:00,543 –> 00:00:03,958 Hello and welcome back to complex analysis.

    2 00:00:04,671 –> 00:00:11,221 and as always, first I want to thank all the nice people that support this channel on Steady, via Paypal or by other means.

    3 00:00:12,014 –> 00:00:20,171 Today in part 8 we will continue talking about the Cauchy-Riemann equations and I will also explain what Wirtinger derivatives are.

    4 00:00:20,786 –> 00:00:27,890 For this as often, we consider a complex function f, that is holomorphic on an open domain U.

    5 00:00:28,771 –> 00:00:36,862 and for such a function we will define a new differential operator as a partial derivative with respect to the variable z.

    6 00:00:37,614 –> 00:00:45,449 Then when we apply this to the function f and look at a given point z_0, this is often called the Wirtinger derivative.

    7 00:00:46,414 –> 00:00:51,694 However, this is not quite correct, because there is also another Wirtinger derivative.

    8 00:00:52,371 –> 00:00:59,075 This one is a little bit strange, because it’s written as the partial derivative with respect to z bar.

    9 00:00:59,700 –> 00:01:06,654 However it turns out that both things here are well defined and indeed useful for our theory.

    10 00:01:07,214 –> 00:01:15,197 For example: for a holomorphic function f this one here should give us the complex derivative of f at the point z_0.

    11 00:01:15,771 –> 00:01:20,477 With this in mind, we will be able to define this new differential operator.

    12 00:01:21,543 –> 00:01:28,172 Also, then we will see that for a holomorphic function, the other Wirtinger derivative should be 0.

    13 00:01:28,986 –> 00:01:35,027 Therefore let’s first concentrate at the Wirtinger derivative on the left hand side. df/dz.

    14 00:01:35,614 –> 00:01:42,105 Now, to make our life a little bit easier let’s write (x + iy) instead of the point z_0.

    15 00:01:42,771 –> 00:01:47,621 So this means, this is the complex derivative of f at the given point.

    16 00:01:47,821 –> 00:01:53,388 Hence it’s just another complex number, which we can write as (a + ib).

    17 00:01:54,571 –> 00:02:02,194 and now this is what we have learned in the last videos. The real part and the imaginary part can be written as partial derivatives.

    18 00:02:03,229 –> 00:02:10,960 This works when we take the real part of the function f as a function u and the imaginary part of f as a function v.

    19 00:02:11,900 –> 00:02:18,438 and then this “a” here is du/dx and b is dv/dx.

    20 00:02:19,743 –> 00:02:26,191 Of course both partial derivatives here are to be evaluated at the given point (x, y).

    21 00:02:26,391 –> 00:02:32,450 However in the upcoming calculation we will omit this point to make everything clearer.

    22 00:02:33,357 –> 00:02:38,070 Of course now in the next step here we want to use the Cauchy-Riemann equations.

    23 00:02:39,271 –> 00:02:45,297 and in order to include them correctly the best thing we can do is to double this complex number here.

    24 00:02:45,943 –> 00:02:51,519 So you see, we add the same number again, but then we have to divide everything by 2.

    25 00:02:52,700 –> 00:02:57,994 and now the idea is, that here in the second part we apply the Cauchy-Riemann equations.

    26 00:02:58,971 –> 00:03:03,171 So instead of du/dx we can write dv/dy.

    27 00:03:03,671 –> 00:03:09,396 and on the other hand dv/dx can be written as -du/dy.

    28 00:03:10,271 –> 00:03:18,933 Ok, what you now should see is that some symmetry is involved. We have 2 partial derivatives with respect to x and 2 with respect to y.

    29 00:03:19,771 –> 00:03:27,665 and this is what we can apply to our knowledge that u is the real part of the function f and v the imaginary part.

    30 00:03:28,429 –> 00:03:35,661 Or in other words we can also look at the map that has (x, y) as an input, but a complex number as an output.

    31 00:03:36,314 –> 00:03:40,795 Namely f at the point (x + iy) should be the output.

    32 00:03:41,243 –> 00:03:47,834 So you see as the map f_R this carries exactly the same information as the map f.

    33 00:03:48,657 –> 00:03:51,139 It’s just again another point of u.

    34 00:03:51,686 –> 00:03:57,282 and this helps us, because this function we can write as u + iv.

    35 00:03:57,482 –> 00:04:02,848 and now you might already see, on the left-hand side we use exactly this.

    36 00:04:03,343 –> 00:04:10,580 Here the first part we can write as the partial derivative with respect to x of u + iv.

    37 00:04:10,971 –> 00:04:17,043 and now in order to get this in the second part as well you see we have to pull out a -i

    38 00:04:17,671 –> 00:04:25,499 and when we do this, you see we also have the partial derivative now with respect to y of u + iv.

    39 00:04:26,871 –> 00:04:31,038 Ok, so this is the result for f’ you really should remember.

    40 00:04:31,614 –> 00:04:38,208 Indeed it’s easier to remember when we use that (u + iv) are essentially the function f.

    41 00:04:38,814 –> 00:04:43,110 Hence we can just define 2 new differential operators.

    42 00:04:44,143 –> 00:04:49,773 Namely we just call the first one df/dx and the second one df/dy.

    43 00:04:50,343 –> 00:04:57,966 and indeed this makes sense. This one is the partial derivative with respect to x, when you see f as this map here.

    44 00:04:58,729 –> 00:05:04,143 Ok, soon I will show you an example and then these 2 operations should be clear.

    45 00:05:04,771 –> 00:05:12,456 However first I want to define the Wirtinger derivatives, because with this calculation here we already know how to do this.

    46 00:05:13,071 –> 00:05:18,456 We can just use combinations of the partial derivatives with respect to x and y.

    47 00:05:19,257 –> 00:05:29,814 Firstly, now when we write d/dz. This should stand for 1/2 times d/dx - i*d/dy.

    48 00:05:30,786 –> 00:05:34,514 So you see, this is exactly what we have learned above.

    49 00:05:35,171 –> 00:05:41,471 The definition makes sense and for a holomorphic function f it gives us the complex derivative of f.

    50 00:05:42,357 –> 00:05:47,022 Ok, now in a similar way we will define d/dz(bar).

    51 00:05:48,357 –> 00:05:56,244 and indeed this should look very similarly. So we also take 1/2 times, and now d/dx + i d/dy.

    52 00:05:56,943 –> 00:06:06,232 We do this, because you see by the Cauchy-Riemann equations, we get 0 when we apply this operation to a holomorphic function f.

    53 00:06:07,243 –> 00:06:11,469 and here please recall. This fits to our motivation from the beginning.

    54 00:06:11,929 –> 00:06:15,627 Ok, then as promised let’s look at an example.

    55 00:06:16,071 –> 00:06:19,976 It shouldn’t be a complicated one. So let’s look at a polynomial.

    56 00:06:20,671 –> 00:06:24,550 Namely we just look at a quadratic function z^2.

    57 00:06:24,957 –> 00:06:29,848 Which means using x and y, we have (x + iy)^2.

    58 00:06:30,500 –> 00:06:41,763 Hence, let’s just expand the square. Which means we have x^2 - y^2 + i2xy.

    59 00:06:42,986 –> 00:06:48,463 Therefore this first part here is our function u and this part is our function v.

    60 00:06:49,029 –> 00:06:57,384 With this we can immediately calculate df/dx, which is here 2x + i2y.

    61 00:06:58,014 –> 00:07:06,883 and similarly we can calculate df/dy, which is here -2y + i2x.

    62 00:07:07,629 –> 00:07:15,551 Now, both things can be simplified when we just use the fact, that (x+iy) is the complex number z.

    63 00:07:16,243 –> 00:07:20,939 Hence, df/dx is simply 2 times z.

    64 00:07:21,857 –> 00:07:27,434 and in a similar way you see for df/dy we have 2 times i times z.

    65 00:07:28,086 –> 00:07:31,638 So we can always use that i^2 is -1.

    66 00:07:32,714 –> 00:07:39,054 Now, with these 2 nice short expressions we should be able to calculate with 2 Wirtinger derivatives.

    67 00:07:40,043 –> 00:07:43,993 So let’s start with the second one, df/dz(bar).

    68 00:07:44,314 –> 00:07:51,736 It’s 1/2 (2z + i times 2iz).

    69 00:07:52,571 –> 00:07:57,384 Again, we know i^2 is -1. So we get 0 here.

    70 00:07:58,229 –> 00:08:02,568 This is what we expected, because the polynomial is a holomorphic function.

    71 00:08:03,300 –> 00:08:07,491 Ok, then in the next step it’s not hard to calculate df/dz.

    72 00:08:07,800 –> 00:08:11,955 It’s the same calculation as before, but now with a minus sign here.

    73 00:08:12,700 –> 00:08:21,545 Therefore in this case, in the parentheses we have (2z + 2z) divided by 1/2, gives us back 2z.

    74 00:08:22,643 –> 00:08:29,352 Also here the result is not so surprising, because it should give us the complex derivative of f.

    75 00:08:29,552 –> 00:08:36,384 Ok, then I would say let’s summarize the video by stating the important fact we have proven here.

    76 00:08:36,886 –> 00:08:40,505 Indeed, this is something one can nicely remember.

    77 00:08:41,486 –> 00:08:53,967 Namely, a complex function f defined on an open domain U is holomorphic if and only if df/dz(bar) is equal to 0 at all points.

    78 00:08:54,471 –> 00:09:01,291 Indeed, this equivalence you already know, because here we simply have hidden the Cauchy-Riemann equations.

    79 00:09:02,214 –> 00:09:10,338 and then in this case as we have proven above the other Wirtinger derivative gives us the complex derivative of f.

    80 00:09:11,071 –> 00:09:15,370 So f’ can be calculated by using df/dz.

    81 00:09:16,100 –> 00:09:22,701 Ok, with this you now know a lot of different differential operators we can use in complex analysis.

    82 00:09:23,714 –> 00:09:28,420 Then I hope that I see you in the next video, where we will talk about power series.

    83 00:09:29,557 –> 00:09:31,594 Have a nice day! Bye!

  • Quiz Content

    Q1: A complex function $f: \mathbb{C} \rightarrow \mathbb{C}$ is holomorphic if and only if

    A1: $$\frac{ \partial f}{\partial z} = 0$$

    A2: $$\frac{ \partial f}{\partial \overline{z}} = 0$$

    A3: $$\frac{ \partial f}{\partial \overline{z}} = \frac{ \partial f}{\partial \overline{z}}$$

    A4: $$\frac{ \partial f}{\partial \overline{z}} = 1$$

    Q2: Consider the complex function $f: \mathbb{C} \rightarrow \mathbb{C}$ given by $f(z) = \overline{z} z $. Which claim is not correct?

    A1: $$\frac{ \partial f}{\partial \overline{z}}(z) = z$$

    A2: $$\frac{ \partial f}{\partial z}(z) = \overline{z}$$

    A3: $$\frac{ \partial f}{\partial z}(z) = \overline{z} z$$

    A4: $$\frac{ \partial f}{\partial z}(0) = 0$$

    A5: $$\frac{ \partial f}{\partial \overline{z}}(0) = 0$$

    Q3: Is the following correct? $$ \frac{ \partial f}{\partial z} = \overline{\frac{ \partial \overline{f}}{\partial \overline{z}}}$$

    A1: Yes!

    A2: No!

  • Last update: 2024-10

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