Find the GCD of the given complex numbers (Gaussian Integers)

[chwala]

Homework Statement

Kindly see attached.

Relevant Equations

Complex Numbers

Hello guys,
I am able to follow the working...but i needed some clarification. By rounding to the nearest integer...did they mean?

$z=1.2-1.4i$ is rounded down to $z=1-i$?

I can see from here they came up with simultaneous equation i.e

$(1-i) + (x+iy) = \dfrac{6}{5} - \dfrac{7i}{5}$ to realise,

$x+iy=\dfrac{1}{5} - \dfrac{2i}{5}$

...the other steps seem clear...any reason why they make use of the so called second summand? One thing i have noted with pure maths- is that its quite easy- the only challenge i seem to use a lot of effort on is understanding the notations and the language used...why did they have to check the other norms i.e for $a$ and $b$ for instance the

$|a|=5^2+14^2$?

...are there cases where the norms may be small...

...I can see gaussian integers in some literature...the bit on the norms is now clearer...Will read on this some more...

cheers.

 

[anuttarasammyak]

Homework Statement:: Kindly see attached.
Relevant Equations:: Complex Numbers

z=1.2−1.4i is rounded down to z=1−i?

Rather than "the nearest integer", I prefer floor function
$$\lfloor x \rfloor$$
, maximum integer that does not exceed x.
$$\lfloor 1.2 \rfloor=1,\ 1.2=1+0.2$$
$$\lfloor -1.4 \rfloor=-2,\ -1.4=-2+0.6$$
$$z=(1-2i)+(0.2+0.6i)$$

 

[chwala]

Rather than "the nearest integer", I prefer floor function
$$\lfloor x \rfloor$$
, maximum integer that does not exceed x.
$$\lfloor 1.2 \rfloor=1,\ 1.2=1+0.2$$
$$\lfloor -1.4 \rfloor=-2,\ -1.4=-2+0.6$$
$$z=(1-2i)+(0.2+0.6i)$$

Looks good! I'll explore more... relatively a new area to me...

 

[WWGD]

$\mathbb Z[ i]$ is the ring of Gaussian Integers, right? It's been a while.

 

[SammyS]

Hello guys,
I am able to follow the working...but i needed some clarification. By rounding to the nearest integer...did they mean?

$z=1.2-1.4i$ is rounded down to $z=1-i$?

That does not correctly state what "they" (the author(s) of the attachment) mean. You shouldn't change the meaning of $z$ like that. The first $z$ corresponds to the complex division of $a$ by $b$, the result being a Gaussian Rational, since $a$ and $b$ are Gaussian Integers. The "rounded" version of $z$ is actually the quotient, $q$, proposed in the attachment. The goal is to find Gaussian Integers, $q, \text{ and, } r,$ so that the expression
$\quad \quad \quad \quad a=q\,b+r$
can be used in the Euclidean Algorithm to get a GDC of $a$ and $b$. Dividing the above equation by $b$ gives us
$\quad \quad \quad \quad \displaystyle \dfrac a b =q+\dfrac r b$

I can see from here they came up with simultaneous equation i.e

$(1-i) + (x+iy) = \dfrac{6}{5} - \dfrac{7i}{5}$ to realise,

$x+iy=\dfrac{1}{5} - \dfrac{2i}{5}$

For the values of $a$ and $b$ used in the attachment, we have:

$\quad \quad \quad \quad \displaystyle \dfrac a b = \dfrac{5+14i}{-4+7i}= \dfrac{6}{5} - \dfrac{7i}{5} = (1-i) + \dfrac{1}{5} - \dfrac{2}{5}i$

So, $q=1-i$ and $\displaystyle \dfrac r b =\dfrac{1}{5} - \dfrac{2}{5}i$

Multiplying $r/b$ by $b$ gives you $r$. Try it.

...the other steps seem clear...any reason why they make use of the so called second summand? One thing i have noted with pure maths- is that its quite easy- the only challenge i seem to use a lot of effort on is understanding the notations and the language used...

The word "summand" may be unusual, but it's not some special word used only in pure maths. You had an equation with the sum of two terms on the right hand side. Each is a summand.

...why did they have to check the other norms i.e for $a$ and $b$ for instance the

$|a|=5^2+14^2$?

...Are there cases where the norms may be small?...I can see gaussian integers in some literature...The bit on the norms is now clearer...Will read on this some more...

cheers.

As to your questions about the Norms: No, you do not generally need to calculate or check the norms for $a$ or $b$ ; not for $r$ or $q$ either. However, the author of your attachment is making a point, saying,

"This just checks that the remainder has small enough Norm (as the rounding method is designed to ensure −− have a think about how it does this). "​

Complex numbers do not have order. It does not make sense to say that $z_1>z_2$ or $z_1<z_2$.

We can't compare $r$ and $b$ directly with "greater than" or ""less than", but we can compare their relative size by comparing Norms. We could just a well use the modulus, but the Norm works out better. The Norm of a Gaussian Integer is itself an Integer − a non-negative Integer.

Added in Edit:
The Euclidean Algorithm gives us a way to find a GCD of two elements of a ring, if such a GCD exists. This algorithm uses Euclidean division iteratively until the at some iteration, the resulting remainder is zero.
So, if the Norm of the remainder decreases for each iteration and since the Norm is a non-negative Integer, the process is guaranteed to terminate in a finite number of steps.