Questions involving complex numbers, that is numbers of the form $a+bi$ where $i^2=-1$ and $a,b\in\mathbb{R}$.

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1answer
32 views

Help with the proof that the sum of all the roots of a complex number is zero

If a complex number $z \neq 0$ has n roots, then each root can be expressed as: $$z_j=(\sqrt[n]{r}) e^{ \left( {i \theta+2\pi j }/{n} \right) } $$ For $j=0,1,2,...,n-1$ Thus, the summation of all ...
1
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1answer
18 views

Complex integrate and residues

Evaluate the integral of that $f(z)=\frac{z+1}{z^2-2z}$ around the circle $|z|=3$ oriented counterclockwise First I found that singularity points are $z=0,z=2$ ...
1
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1answer
16 views

Complex analysis, residues

Find the residue at $z=0$ of $f(z)=\dfrac{\sinh z}{z^4(1-z^2)}$. I did \begin{align} \frac{\sinh z}{z^4(1-z^2)} & =\frac{1}{z^4}\left[\left(\sum_{n=0}^\infty ...
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1answer
12 views

Question about a simple rule of the complex logarithm

According to the Wikipedia page on complex logarithms: Also, the identity $\log(xy) = \log x + \log y$ can fail: the two sides can differ by an integer multiple of $2\pi i$. Does the same hold ...
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1answer
31 views

Complex Numbers and their Conjugates

How many complex numbers are there that are conjugates of their own cubes? Is there some simple way to find this?
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2answers
292 views

Mathematical Way To find how many complex numbers

Suppose that $a$ and $b$ are integers and that $ |a + bi| \leq 5 $, then how many complex numbers $a + bi$ are there? Is there a mathematical way to do this?
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1answer
30 views

$a \exp(iwt)+b \exp(-iwt)=0 \Rightarrow a = b = 0$

Suppose $$a \exp(iwt)+b \exp(-iwt)=0$$ How do we show that $a=b=0$?
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2answers
49 views

What does this circle look like?

I have been given an integral to evaluate over a circle $$|z-i|=3$$ I am trying to work out what this circle looks like. Is the radius of the circle $3$ centred on the origin?
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0answers
16 views

how to find singularities of complex functions.

how to find singularities of this function; i) $f(z)= 1+\frac{1}{\sqrt{z}}$ and ii) $f(z)=e^{1+\frac{1}{\sqrt{z}}}$ As the z is in the denominator the function (i) has pole at z=0 but what's ...
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2answers
81 views

The Imaginary number [duplicate]

Sometimes one writes $i^2=-1$ to characterize the imaginary number and sometimes as the root of $-1$. So when I resolve the first equation, I get for the imaginary number two roots of $-1$ and thus ...
1
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1answer
67 views

$e^{2\pi i x} = (e^{2\pi i})^x$: What happens if x is rational? [duplicate]

I'm a bit embarrassed that I've had difficulty on getting around this one: $$e^{2\pi i x}$$ Solving it by itself, we can reduce it down to $(e^{2\pi i})^x = 1^x$ such that $e^{2\pi i x} = 1$ for all ...
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0answers
26 views

Complex vector identity

Let $f=(f_1,f_2,f_3)$ be a complex vector. Can we see that $$G:=\frac{2\Im(f_2\bar{f_3})+i2\Im(f_3\bar{f_1})}{|f|^2-2\Im(f_1\bar{f_2})}=\frac{f_3}{f_1-if_2}$$ I tried using $f_j=x_{j,u}-ix_{j,v}$ ...
1
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1answer
23 views

Norm of a complex cross product

Let $c=(c_1,c_2,c_3)$ be a complex vector. How can we see that $\|c\|^2=\|c\times \bar{c}\|$? Here the bar means component wise complex conjugation, the norm is the Hermitian norm, and the cross ...
2
votes
2answers
36 views

Complex Number, Quaternions and Octonions [duplicate]

There are complex $\mathbb C$, quaternions $\mathbb H$ and octonions $\mathbb O$. Is there any higher dimensional generalization of them, such in the $\mathbb R^{16}$? Or why do we just study three ...
5
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1answer
51 views

Real Manifold … Complex Coordinates?

I'm working in an earlier edition of John Lee's book on smooth manifolds, and he has a number of problems where he represents a real manifold using complex variables. For instance in chapter 3 ...
0
votes
1answer
21 views

What would the multifunctional inverse of $F(x)=|x|$ be?

What would the multifunctional inverse of $F(x)=|x|$ be, assuming $x$ is on the complex plane. Also, how would this usually be represented? Note that this won't be a 'true' function. (But assume a ...
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3answers
59 views

Complex numbers? [duplicate]

There are plenty of questions out there asking what complex numbers mean and I never seem to get any of them. I have a few specific questions i want to ask about complex numbers. 1) what is the ...
0
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1answer
34 views

Proving relation for square root of complex number

How do I represent $\sqrt{1 + ja}$? I'm trying to show that it's approximately equal to $\pm(1 + \frac{ja}{2})$ when $a \leq 1$.
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2answers
28 views

Ignoring the pole?

I have the integral $$\int^{2 \pi}_0 \frac{z}{z+2} dz$$ where $$|z|=3$$ I parametrise the integral and get $$\int^{2\pi}_0 \frac{9 ie^{2i\theta}}{3e^{i\theta}+2}$$ and this gives the required answer ...
0
votes
1answer
24 views

Evaluating a complex integral in punctured plane.

I am trying to evaluate the complex integral $$\int \frac{z}{z+2} dz$$ And $$|z|=3$$ We can see there is a pole at $z=-2$. How do I go about solving this, what is the strategy?
1
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1answer
28 views

Arithmetic progression with complex common difference?

Suppose we have the following sequence: $$\{0,i,2i,3i,4i,5i\}$$ Can we call this sequence an arithmetic progression with first term $0$ and common difference of $i$ ? Clarification: Here, $i$ is ...
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2answers
33 views

Is there such a thing as complex rational numbers and does it have the same properties as the usual complex numbers as extension of the real numbers?

I've been wondering if there is any use to defining a set that is isomorphic to $\mathbb{Q}^2$ (in the same way that $\mathbb{C}$ is isomorphic to $\mathbb{R}^2$). I immediately see a problem with ...
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0answers
17 views

Find a generator for vectorial subspace

S = {$(a, b, c, d) ∈ C^4 : 2ia = b, c + d − ib = 0$} $c+d-i(2ia)=0$ $c+d+2a=0$ $c=-d-2a$ $(a,2ia,-2a-d,d)=a(1,2i,-2,0)+d(0,0,-1,1)$ Is this solution correct?
3
votes
3answers
61 views

Why imaginary numbers axis is plotted perpendicular to the real numbers axis?

Negative numbers axis is plotted to the opposite side of the positive real number axis that make sense but i do not understand why imaginary numbers are plotted perpendicular to the real numbers axis. ...
2
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4answers
249 views

Cauchy integral formula

Can someone please help me answer this question as I cannot seem to get to the answer. Please note that the Cauchy integral formula must be used in order to solve it. Many thanks in advance! ...
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0answers
38 views

Complex function

Can anyone give me a hint to approach this question? I haven't done anything like this before so I'm bit confused about what this question is asking. Thank you very much for all your help.
0
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1answer
35 views

Midpoint of two complex numbers in polar form

Say we have two complex numbers: $re^{i\theta}$ and $se^{i\phi}$ Is there a straightforward way to find the polar form of the midpoint of these two complex numbers? I think I'm correct in saying ...
2
votes
2answers
52 views

Help solve ${{z}^{3}}=\overline{z}$ ($z\in \mathbb{C}$) [duplicate]

Me and my friend try to solve $${{z}^{3}}=\overline{z}$$ where $z \in \mathbb{C}$. My way to solve it was: $\operatorname{cin}(\theta )=\cos(\theta)+\sin(\theta)i$ \begin{align} & z=r ...
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3answers
18 views

Electrical Engineering (complex numbers)

Electrical Engineering ($j=i=\sqrt{-1}$): $$H_v(\omega)=\frac{R}{R+\frac{1}{j\omega C}}=\frac{j\omega CR}{1+Rj\omega C}$$ And we know that: $\omega_0=\frac{1}{RC}\Longleftrightarrow ...
1
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3answers
21 views

Complex plane (Show that triangle is right-angled)

The points $O$,$P$ and $Q$ in the complex plane represent the complex numbers $0+0i$, $4+2i$ and $3-i$ respectively. Find the exact length of $PQ$ and hence, or otherwise, show that triangle $OPQ$ is ...
1
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1answer
32 views

Complex Number (Angle)

The complex number $z$ is given by $z=-2+2i$ Find the modulus and argument of $z$ Write down the modulus and argument of $\frac{1}{z}$ Show on an Argand diagram the points A,B and C representing the ...
0
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1answer
30 views

Complex Numbers (Find p and q)

The complex numbers z1 and z2 are given by $$z_1=5+i,z_2=2-3i$$ Determine the values of the real constants $p$ and $q$ such that $$\frac{p+iq+3z_1}{p-iq+3z_2}=2i$$ My attempt, I substitute $z_1$ and ...
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3answers
110 views

Complex Numbers (Modulus)

The complex numbers $z_1$ and $z_2$ are given by $$z_1=5+i,z_2=2-3i$$ Find the modulus of $z_1-z_2$ My attempt, modulus of $z_1-z_2=\sqrt{5^2+1^2}-\sqrt{2^2+3^2}$ $=\sqrt{26}-\sqrt{13}$ ...
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1answer
21 views

Derivatives of real part of function

My physics textbook gives me the complex form equation of simple harmonic motion as: $$z = Ae^{i(\omega _{o}t+\phi )}$$ and then defines $$ x = Re (z) $$ From there they argue that $$ \frac{\partial ...
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1answer
27 views

Explanation of two argument variant for arctan

Can someone please explain why $$\tan^{-1}\left(\frac{y}{x}\right)$$ has the additional conditions based on what the value of x and y are? I'm most specifically interested in the second equation: ...
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3answers
41 views

Cube roots of complex numbers [closed]

I need help with finding the cube roots of the complex number 27... I know that the obvious answer is three, but what is the less simple method to solving this?
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0answers
13 views

Simplifying complex exponential

Two simplifications from my book that I don't understand, first: 5exp(-j1.571) = - j5 Why does the real part get dropped off? Also: exp(j3.785) = -exp(0.643) Is there a way to directly covert ...
1
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2answers
38 views

Using Eulers formula

I am trying to figure out how \begin{equation*} e^{i(-1+i\sqrt{3})}=e^{-\sqrt{3}} (cos(1)-i sin(1))?? \end{equation*} I know that Euler's formula states that \begin{equation*} e^{ix} = \cos(x) + i ...
0
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1answer
23 views

Evaluating complex integral on circle

I am trying to evaluate the integral $$\int \frac{2z-1}{z(z-1)} dz$$ counter clockwise around the circle $$|z|=2$$ First we apply partial fraction decomposition to get $$\int \frac{1}{z}+\int ...
0
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2answers
18 views

Complex number (Rhombus)

Given that $z_1=1+2i$ and $z_2=\frac{3}{5}+\frac{4}{5}i$, write $z_1z_2$ and $\frac{z_1}{z_2}$ in the form $p+iq$, where $p$ and $q \in R$. In an Argand diagram, the origin O and the points ...
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1answer
29 views

Sum of complex series

After stating the sum I wrote z in polar form and then proceeded to calculate the real part of the sum I stated in the first part. However the working got tedious very soon and I was not able to ...
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1answer
22 views

Argument of Complex Number (Am I wrong?)

I'm given $z=-2+\sqrt{3}i$. So I worked out the argument of $arg(z)=\tan^{-1}(\frac{\sqrt{3}}{-2})$. I got the answer $2.256$rad. But the given answer is $2.45$rad. Am I wrong?
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0answers
13 views

Integrating complex functions over the unit circle

I am trying to evaluate $$\int_c \bar z dz$$ where the contour is the unit circle. I know the limits of $\theta$ is $0 \to 2\pi$ How do I get to the answer of $2\pi i$?
4
votes
3answers
66 views

Finding local max of analytic function

Given a function $f=z^2+iz+3-i$. I need to find the the maximum of $|f(z)|$ in the domain $|z|\leq 1$ I know that the maximimum should be on $|z|=1$ but when I tried to put $z=e^{i\theta} $ in the ...
1
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2answers
17 views

Simplifying Complex numbers

Help me simplify this complex number: Hints are welcome, so that I can see how to move on $$\left(\frac{1+6i}{\sqrt{76}e^{\frac{1}{2}\pi i}}\right)^{2i}$$
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3answers
39 views

Complex number $\tan \alpha+i$

Given that $z=\tan \alpha+i$, where $0<\alpha<\frac{1}{2}\pi$ Find $\left |z \right |$. I've never seen this kind of example in my book. Can anyone guide me? Thanks a lot. How to find $arg ...
3
votes
2answers
181 views

Square roots of Complex Number. [duplicate]

Calculate, in the form $a+ib$, where $a,b\in \Bbb R$, the square roots of $16-30i$. My attempt with $(a+ib)^2 =16-30i$ makes me get $a^2+b^2=16$ and $2ab=−30$. Is this correct?
1
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1answer
32 views

Find $\int_c \bar z$ along the parabola $y=x^2$ from $(0,0)$ to $(1,1)$

I know $\bar z=x-iy$ So we have $$\int_c x-iy \,dz$$ when split up gives us $$\int^1_0 x \, dx-i\int^1_0 x^2 \cdot 2x \, dx$$ and then I integrate as usual as usual and I get the result ...
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2answers
23 views

equation of a line into the complex form

So if i am given an equation of a line in complex form for example $Re|(1+i)z| = 0$, I could turn this into its real counter part on the x-y plane and graph it. Is there a way to go in the other ...
0
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0answers
27 views

complex variable inequality

Let $B$ and $C$ be nonegative real numbers and $A$ a complex number. Suppose that $$ 0\leq B-2Re(\overline{\lambda}A) + |\lambda|^2 C \ \forall \ \lambda \in \mathbb{C} $$ Conclude that $|A|^2 \leq ...