Questions involving complex numbers: numbers of the form $a+bi$ where $i^2=-1$.

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23
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9answers
23k views

What is $\sqrt{i}$?

If $i=\sqrt{-1}$, is $\large\sqrt{i}$ imaginary? Is it used or considered often in mathematics? How is it notated?
2
votes
2answers
28 views

summing the powers of a complex number

Let $z=e^{\frac{2\pi i}{5}}$, then $1+z+z^2+z^3+5z^4+4z^5+4z^6+4z^7+4z^8+5z^9=?$ I am kind of confused since by drawing a graph, $1+z+z^2+z^3+z^4$ should be zero, but using computational softwares ...
0
votes
0answers
39 views

Subring of $\mathbb{Z}[i]$ and an infinite set $X$ such that $\exists x \forall y \in X \,\,x^2\mid y^2$ but $\forall x \forall y \,\,x \not\mid y$

This is a question derived from A subring of the ring of Gaussian integers such that $a^2 \mid b^2$ does not lead to $a\mid b$ in infinitely many such cases. Is there a subring $R$ of Gaussian ...
0
votes
1answer
29 views

Finding equations for plane figures using complex coordinates

I have to find conditions defining the following plane figures: Where: $a=3$ and $b=7$ I know that circumference form is: $$\left |z-z_0 \right | =b$$ So, for c. with center $(3,3)$ and radius ...
0
votes
1answer
33 views

the absolute value of $\frac{1}{e^{i\omega t}-1}$

I am told to get the absolute value of $$\frac{1}{e^{i\omega t}-1}$$ I sense that there's something ridiculously simple about this, but I tried working from the fact that if I square it, the absolute ...
3
votes
1answer
57 views

Let $f(z) = z + z^2$ and let $V = \displaystyle \{z \in \mathbb{C} : |z| < \frac{1}{2}, \frac{3\pi}{4} < arg\{z\} < \frac{5\pi}{4}\}$.

Let $f(z) = z + z^2$ and let $V = \displaystyle \{z \in \mathbb{C} : |z| < \frac{1}{2}, \frac{3\pi}{4} < arg\{z\} < \frac{5\pi}{4}\}$. $(a)$ Show that $f(V) \subset V.$ $(b)$ Let $f_n$ be ...
1
vote
0answers
35 views

Weird conformal map problem

Construct a conformal map from the region $\omega$ = open disk of radius 1 centered at 0 minus the closed disk of radius 0.5 centered at 0.5 to $\mathbb{D}$ = disk radius 1 centered at 0. I really ...
0
votes
1answer
49 views

How to find the principal argument of $z^4$, given $z$?

I am having trouble with a homework question. Let $ z= \cos\left(\frac{3}{4}\pi\right)+i \sin\left(\frac{3}{4}\pi\right)$. What is the principal argument of $z^4$ in radians? Is it undefined? If ...
0
votes
3answers
85 views

Why does this work for $ i^{2i} $?

I'm finding the principal value of $$ i^{2i} $$ And I know it's solved like this: $$ (e^{ i\pi /2})^{2i} $$ $$ e^{ i^{2} \pi} $$ $$ e^{- \pi} $$ I understand the process but I don't understand ...
0
votes
3answers
121 views

How is $ i^{-1} = -i$ and $i^{-3} = i$?

Now I know that with positive powers of $i$ the cycle is: $i , -1 , -i , 1\ldots$ The negative power cycle is: $-i , -1 , i , 1 \ldots$ Can someone explain to me how $\frac 1 {\sqrt{-1}}$ is equal ...
0
votes
1answer
31 views

How can I find $x$, $y$ values for $\frac{(1+i)x-2i}{3+i}+\frac{(2-3i)y+i}{3-i}=i$

$$ \frac{(1+i)x-2i}{3+i}+\frac{(2-3i)y+i}{3-i}=i $$ I believe the format I need in order to solve this problem should be such that the real parts and imaginary parts are separated, ...
1
vote
1answer
64 views

Polar form of the sum of complex numbers $\operatorname{cis} 75 + \operatorname{cis} 83 + \ldots+ \operatorname{cis} 147$

The number $\operatorname{cis} 75 + \operatorname{cis} 83 + \operatorname{cis} 91 +\dots+ \operatorname{cis} 147$ is expressed in the form $r\operatorname{cis}(\theta)$, where $0\leq \theta< ...
1
vote
1answer
46 views

Why is a complex number plus infinity equal to infinity?

Why is $$2 + 3 i + \infty = \infty$$ according to Mathematica and Wolfram Alpha? Shouldn't it be: $$2 + 3 i + \infty = \infty + 3 i$$ ? After all: $$2 + 3 i + 10 = 12 + 3 i$$ and not: $$2 + 3 i ...
1
vote
0answers
30 views

Roots of Unity: Different Methods

I am aware of two different 'methods' for finding say the cube roots of 1. They are Let $z^3=1$ and $z=R(\cos \theta + i\sin\theta)$, then use DMT and equate terms to find that $R=1$, ...
0
votes
1answer
24 views

Principal angle and Euler form of cube root of unity.

The principal angles of the complex arguments are in the range $-\pi \le x\le \pi $. If so why does the comlex number $\omega =\frac{\left(-1-\sqrt{3}i\right)}{2}$ is represented by ...
0
votes
1answer
38 views

Finding complex roots of integer polynomials

How would one find approximates for complex root of polynomial with integer coefficients,I know for example the Newton's method $$x_n=x_{n-1}-\frac{f(x_{n-1})}{f'(x_{n-1})}$$ Anyway is it possible to ...
2
votes
2answers
42 views

Difference between the complex roots of $f(x)$ and $|f(x)|^2$

I suppose a basic question, but it's causing me more problems than I envisioned! I have some polynomial $f(x)$ for which the roots are complex, $x+iy$. How will these roots change if I now take ...
0
votes
2answers
61 views

Failure of De Moivre's Theorem

I know that De Moivre's Theorem does not necessarily work for non-integer powers. The classic counter-example is by considering $\left (\cos \theta + i \sin \theta \right )^n=\cos n\theta + i \sin n ...
2
votes
4answers
54 views

Writing the complex number $z = 1 - \sin{\alpha} + i\cos{\alpha}$ in trigonometric form

Now I can't finish this problem: Express the complex number $z = 1 - \sin{\alpha} + i\cos{\alpha}$ in trigonometric form, where $0 < \alpha < \frac{\pi}{2}$. So the goal is to determine both ...
0
votes
2answers
79 views

Can I approximate a complex number by its imaginary part, if real part is small compared to imaginary part?

I have the following doubt. How do you explain this? Here $j$ means $\sqrt{-1}$.
0
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1answer
26 views

Calculating $\sum_{n=0}^\infty (r e^{2 \pi i \alpha})^{n!}$ for $\alpha \in \mathbb Q$.

I need to calculate $\sum_{n=0}^\infty (r e^{2 \pi i \alpha})^{n!}$ for $\alpha \in \mathbb Q$ and $r \in \mathbb R$. My Attempt: $\sum_{n=0}^\infty (r e^{2 \pi i \alpha})^{n!}=\sum_{n=0}^\infty r ...
0
votes
1answer
42 views

Construction of Hyper-Complex Numbers

How does one construct a hyper-complex number multiplication table? For example: Quarternions: ...
2
votes
1answer
32 views

Equality of complex numbers

I'm currently reading some notes on Complex Numbers and came across this 'proof' regarding the equality of complex numbers. Claim: Two complex numbers $a+bi$ and $c+di$ are equal iff $a=b$ and $c=d$, ...
0
votes
2answers
31 views

Is ring of Gaussian rationals in unique factorization domain?

Instead of Gaussian integers, let us think about Gaussian rationals, where $a$ and $b$ in $a+bi$ are rational numbers. Then would ring of Gaussian rationals be in unique factorization domain?
1
vote
2answers
38 views

rationalize the complex number multiplication rule

For a middle school student without previous knowledge of complex number, how do one introduce the multiplication rules of complex number? i.e., if we have two real number pairs of $(a,b)$ and ...
0
votes
1answer
26 views

Complex numbers - locii

I have been asked to solve the following and represent the answer graphically: A) $| \arg z - (\pi/4) | < (\pi/2)$ I understand that this means the difference between the argument of $z$ and ...
-2
votes
1answer
55 views

Polar Coordinate usind De Moivre’s Theorem

I need the solutions for the following problem: Find all solutions over $\mathbb{C}$ to the equation $x^3=i^2$. I tried using De Moivre’s Theorem can't get around it. Note: The question originally ...
1
vote
1answer
55 views

square of complex numbers

I have this equation from here: but it is not equal to: $$(a + bi)^2 = a^2 + 2abi + (bi)^2.$$ could someone explain me what is the difference between this two calcultion?
1
vote
1answer
163 views

arg(z) vs. Arg(z)

What is the difference between the arg(z) and the Arg(z), where z is a complex number of the form a+bi for example z = -2 - 2i the angle from the positive ...
5
votes
0answers
139 views

Convergence of infinite series of complex numbers [duplicate]

This has been bugging me for some months since our lecturer, a fields medalist, mentioned that he couldn't solve it when he was our age, yet had had two students submit solutions to it (during our ...
2
votes
1answer
48 views

Factoring a complex polynomial

Factorize the polynomial : $$ p(x) = x^{5} - x^{4}+ 4x - 4 $$ In real factors in the lowest degree possible. So in previous questions I have been given at least one rot so that I can factorize it ...
1
vote
3answers
66 views

Complex Equations

The Equation: $$ z^{4} -2 z^{3} + 12z^{2} -14z + 35 = 0 $$ has a root with a real part 1, solve the equation. When it says a real part of 1, does this mean that we could use (z-1) and use ...
0
votes
1answer
33 views

Arg(z) from $z^n$

$z \in \mathbb{C}$. If the principal argument of $z^n$ is in the quadrant $q$, what is the complete set of values for $Arg(z)$? For example if $n = 3$ and $q = 2$, how could I find all values of ...
9
votes
2answers
219 views

History of the matrix representation of complex numbers

It is well-known to many that $\mathbb{C}$ can be represented by matrices of the form $\left[ \begin{array}{cc} a & b \\ -b & a \end{array} \right]$. For example, see this question or this ...
0
votes
1answer
41 views

Equilateral triangle from complex numbers

We know that $z_1+z_2+z_3=0$ and $|z_1|=|z_2|=|z_3|=1$ where $z_1,z_2,z_3$ are complex numbers. How can we show that the images of $w_1=z_1^4*z_2^3 , w_2=z_2^4*z_3^3 , w_3=z_3^4*z_1^3$ form an ...
5
votes
2answers
78 views

Complex Numbers - Finding Roots

Hi there I was wondering if someone could help me? I am struggling to find the roots of the polynomial $z^4+2z+3=0$ It is not a quadratic so can't use the quadratic formula so am not quite sure ...
10
votes
1answer
218 views

Interpret to a complex plane!

$\newcommand{\Re}{\operatorname{Re}}\newcommand{\Im}{\operatorname{Im}}$The question is: Interpret $$ \Re z + \Im z = 1 $$ geometrically in the complex plane. Writing $z = x + yi$, the condition ...
2
votes
2answers
54 views

Another way to solve this problem with complex expressions

The problem is this: Express $x$ and $y$ with $u$ and $v$, if $\dfrac{1}{x+iy} + \dfrac{1}{u+iv} = 1$ Where $x,y,u,v \in \mathbb{R}$, and $i^2 = -1$. I could solve it, but I used a hairy and ...
4
votes
2answers
223 views

Problem getting the real roots of this complex expression

I'm trying to get the real roots of this expression: $$\dfrac{1}{z-i}+\dfrac{2+i}{1+i} = \sqrt{2}$$ Where $i^2=-1$ and $z=x+iy$. I tried to simplify that with Algebra, and then separate the real ...
3
votes
3answers
103 views

Problems with trigonometry getting the power of this complex expression

I'm here because I can't finish this problem, that comes from a Russian book: Calculate $z^{40}$ where $z = \dfrac{1+i\sqrt{3}}{1-i}$ Here $i=\sqrt{-1}$. All I know right now is I need to use ...
5
votes
1answer
82 views

If $e^{i\theta}=e^{i\varphi}$, then $\theta-\varphi=2k\pi$

This is pretty easy I think but I am having a tough time trying to prove this in a satisfying way to me. I am trying to show that $$e^{i\theta}=e^{i\varphi} \Rightarrow \theta-\varphi=2k\pi,\, \text{ ...
3
votes
2answers
81 views

Find all complex number $z\in\Bbb{C}$ such that $\vert z\vert=\vert z^{-1}\vert=\vert z-1\vert$

Find all complex number $z\in\Bbb{C}$ such that $$\vert z\vert=\vert z^{-1}\vert=\vert z-1\vert$$ I tried to write $z=a+ib$, clearly $z=1$ is not a solution. I have to solve $$\left\{ ...
0
votes
1answer
23 views

Factor complex equation

Im having some difficulty in factoring the following complex equation. The image bellow is taken from WolframAlpha, can anyone explain how I can factor this equation. In the task I am told one ...
0
votes
2answers
58 views

Can the cube of 2 different complex numbers be the same?

Can the cube of 2 different complex numbers be the same? I think it cannot be the same, but I don't really know how to prove it. I tried to expand it but it gives a very ugly result.
1
vote
0answers
32 views

Computing the tangential and cross components of one quantity using gnomonic projection

I have a spin-2 field given called shape distortion of galaxies as $$\gamma=\gamma_1+i\gamma_2=|\gamma|e^{-2i\phi}$$ where $\phi$ is the orientation angle. If this quantity has been measured on ...
15
votes
9answers
1k views

Is there an interval notation for complex numbers?

Just as $$\{x \in \mathbb{R}: a \leq x \leq b\}$$ can be written in the more-compact form $[a,b],$ is there an analogous notation for $$\{z \in \mathbb{C}:z=x+yi, x \in[a,b], y \in[c,d]\} \quad ?$$ ...
1
vote
1answer
57 views

Sum of the trigonometric series

I'm studying de Moivre's theorem's application on the summation of trigonometric series. Here's what I have so far: \begin{align*} \sum_{k=0}^n \cos(k\theta)&= \text{Re}\sum_{k=0}^n e^{ki\theta} ...
0
votes
2answers
45 views

How to draw the Bode diagram for a given transfer function?

With this transfer function: $$G(s)=\displaystyle\frac{10(s+1)}{s(0.1s+1)}$$ I need to do operations to draw the Bode diagram manually I have this: $G(jw)=\displaystyle\frac{10jw+10}{-0.1w^2+jw}$ ...
1
vote
2answers
74 views

Using complex solutions in a factorisation

I'm working through an assignment, and have become stuck understanding the question... In part (a) I am asked to solve the equation: $z^5 = -1$ I have done this, so I now have a set of solutions: ...
3
votes
3answers
290 views

The limit of complex sequence

$$\lim\limits_{n \rightarrow \infty} \left(\frac{i}{1+i}\right)^n$$ I think the limit is $0$; is it true that $\forall a,b\in \Bbb C$, if $|a|<|b|$ then $\lim\limits_{n\rightarrow ...