How do I prove the multiplicative inverse in complex arithmetic?
$begingroup$
I am working with Sheldon Axler's Linear Algebra Done Right and I could use some help from you guys. I have the following property of complex arithmetic:
for every $alpha in mathbb{C}$ with $alpha neq$ 0, there exist a >unique $beta in mathbb{C}$ such that $alphabeta = 1$
I want to prove this. I start by using de definition of multiplication of complex number.
$(a + bi)(c + di) = (ac - bd)+(ad+bc)i$
And the fact that:
$1 = (1 + 0i)$
Aaaand I run out of ideas. I really have no idea on how to continue so a clue would be greatly appreciated.
complex-numbers
$endgroup$
add a comment |
$begingroup$
I am working with Sheldon Axler's Linear Algebra Done Right and I could use some help from you guys. I have the following property of complex arithmetic:
for every $alpha in mathbb{C}$ with $alpha neq$ 0, there exist a >unique $beta in mathbb{C}$ such that $alphabeta = 1$
I want to prove this. I start by using de definition of multiplication of complex number.
$(a + bi)(c + di) = (ac - bd)+(ad+bc)i$
And the fact that:
$1 = (1 + 0i)$
Aaaand I run out of ideas. I really have no idea on how to continue so a clue would be greatly appreciated.
complex-numbers
$endgroup$
add a comment |
$begingroup$
I am working with Sheldon Axler's Linear Algebra Done Right and I could use some help from you guys. I have the following property of complex arithmetic:
for every $alpha in mathbb{C}$ with $alpha neq$ 0, there exist a >unique $beta in mathbb{C}$ such that $alphabeta = 1$
I want to prove this. I start by using de definition of multiplication of complex number.
$(a + bi)(c + di) = (ac - bd)+(ad+bc)i$
And the fact that:
$1 = (1 + 0i)$
Aaaand I run out of ideas. I really have no idea on how to continue so a clue would be greatly appreciated.
complex-numbers
$endgroup$
I am working with Sheldon Axler's Linear Algebra Done Right and I could use some help from you guys. I have the following property of complex arithmetic:
for every $alpha in mathbb{C}$ with $alpha neq$ 0, there exist a >unique $beta in mathbb{C}$ such that $alphabeta = 1$
I want to prove this. I start by using de definition of multiplication of complex number.
$(a + bi)(c + di) = (ac - bd)+(ad+bc)i$
And the fact that:
$1 = (1 + 0i)$
Aaaand I run out of ideas. I really have no idea on how to continue so a clue would be greatly appreciated.
complex-numbers
complex-numbers
edited Dec 28 '18 at 5:30
Eric Wofsey
189k14216347
189k14216347
asked Apr 14 '18 at 9:18
A39-A20A39-A20
206
206
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Basic operations: $;alpha:=x+iyneq0iff x^2+y^2neq0;$ ( since $;x,yinBbb R;$) , so:
$$alphaalpha^{-1}=1impliesalpha^{-1}=frac1alpha=frac1{x+iy}cdotfrac{x-iy}{x-iy}=frac x{x^2+y^2}-frac y{x^2+y^2}i$$
and there you have a cartesian expression for $;alpha^{-1};$ whenever $;0neqalphainBbb C;$
$endgroup$
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
add a comment |
$begingroup$
You start from $(a+bi)(c+di) = (ac-bd)+(ad+bc)i$. Then impose the conditions $ac-bd = 1$ and $ad+bc=0$. From the second you get $d = -lambda b$ and $c = lambda a$ for some $lambda$. Plugging these in the first equation you end up with $lambda = frac{1}{a^2+b^2}$ and so $c = frac{a}{a^2+b^2}, d = -frac{b}{a^2+b^2}$. So your inverse is
$$beta = frac{a}{a^2+b^2}-ifrac{b}{a^2+b^2}.$$
$endgroup$
add a comment |
$begingroup$
You have the following:
$ac-bd=1$, Eq. 1
$bc+ad=0$, Eq. 2
Recognize this as a linear system of two equations in two unknowns $c, d$ and solve using linear combinations. But before you do so check the matrix of known coefficients:
$left[begin{matrix}a&-b\b&aend{matrix}right]$
Observe that this guarantees a unique solution if and only if the determinant $a^2+b^2$ is nonzero, but for real numbers $a$ and $b$ this requirement is met unless $a=b=0$; the square of one nonzero number is (strictly) positive and the square of the other real number is never negative. Thereby, a unique solution is guaranteed for any nonzero complex number input $a+bi$.
Now for a linear combination solution. Setting the multipliers to make the terms containing $d$ cancel gives:
$a^2c-abd=a$, Eq. 1 × $a$
$b^2c+abd=0$, Eq. 2 × $b$
And then the sum contains only one unknown:
$(a^2+b^2)c=a, c=frac{a}{a^2+b^2}$
Substitute this for $c$ in either Eq. 1 or Eq. 2 and you can then solve for $d=-frac{b}{a^2+b^2}$.
$endgroup$
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Basic operations: $;alpha:=x+iyneq0iff x^2+y^2neq0;$ ( since $;x,yinBbb R;$) , so:
$$alphaalpha^{-1}=1impliesalpha^{-1}=frac1alpha=frac1{x+iy}cdotfrac{x-iy}{x-iy}=frac x{x^2+y^2}-frac y{x^2+y^2}i$$
and there you have a cartesian expression for $;alpha^{-1};$ whenever $;0neqalphainBbb C;$
$endgroup$
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
add a comment |
$begingroup$
Basic operations: $;alpha:=x+iyneq0iff x^2+y^2neq0;$ ( since $;x,yinBbb R;$) , so:
$$alphaalpha^{-1}=1impliesalpha^{-1}=frac1alpha=frac1{x+iy}cdotfrac{x-iy}{x-iy}=frac x{x^2+y^2}-frac y{x^2+y^2}i$$
and there you have a cartesian expression for $;alpha^{-1};$ whenever $;0neqalphainBbb C;$
$endgroup$
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
add a comment |
$begingroup$
Basic operations: $;alpha:=x+iyneq0iff x^2+y^2neq0;$ ( since $;x,yinBbb R;$) , so:
$$alphaalpha^{-1}=1impliesalpha^{-1}=frac1alpha=frac1{x+iy}cdotfrac{x-iy}{x-iy}=frac x{x^2+y^2}-frac y{x^2+y^2}i$$
and there you have a cartesian expression for $;alpha^{-1};$ whenever $;0neqalphainBbb C;$
$endgroup$
Basic operations: $;alpha:=x+iyneq0iff x^2+y^2neq0;$ ( since $;x,yinBbb R;$) , so:
$$alphaalpha^{-1}=1impliesalpha^{-1}=frac1alpha=frac1{x+iy}cdotfrac{x-iy}{x-iy}=frac x{x^2+y^2}-frac y{x^2+y^2}i$$
and there you have a cartesian expression for $;alpha^{-1};$ whenever $;0neqalphainBbb C;$
answered Apr 14 '18 at 9:32
DonAntonioDonAntonio
179k1494233
179k1494233
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
add a comment |
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
1
1
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
$begingroup$
Thank you for your help! What you did in the beginning by saying that $alpha^-1 = beta$ made it a lot easier for me to understand. I just want to be sure that I understand the proof: $alphaalpha^-1 = 1 Rightarrow alpha^1 = frac{1}{alpha} = frac{1}{x+iy}cdot1 Rightarrow frac{1}{x+iy}cdotfrac{x-iy}{x-iy}$ Since $1 = frac{x-iy}{x-iy}$
$endgroup$
– A39-A20
Apr 15 '18 at 8:00
add a comment |
$begingroup$
You start from $(a+bi)(c+di) = (ac-bd)+(ad+bc)i$. Then impose the conditions $ac-bd = 1$ and $ad+bc=0$. From the second you get $d = -lambda b$ and $c = lambda a$ for some $lambda$. Plugging these in the first equation you end up with $lambda = frac{1}{a^2+b^2}$ and so $c = frac{a}{a^2+b^2}, d = -frac{b}{a^2+b^2}$. So your inverse is
$$beta = frac{a}{a^2+b^2}-ifrac{b}{a^2+b^2}.$$
$endgroup$
add a comment |
$begingroup$
You start from $(a+bi)(c+di) = (ac-bd)+(ad+bc)i$. Then impose the conditions $ac-bd = 1$ and $ad+bc=0$. From the second you get $d = -lambda b$ and $c = lambda a$ for some $lambda$. Plugging these in the first equation you end up with $lambda = frac{1}{a^2+b^2}$ and so $c = frac{a}{a^2+b^2}, d = -frac{b}{a^2+b^2}$. So your inverse is
$$beta = frac{a}{a^2+b^2}-ifrac{b}{a^2+b^2}.$$
$endgroup$
add a comment |
$begingroup$
You start from $(a+bi)(c+di) = (ac-bd)+(ad+bc)i$. Then impose the conditions $ac-bd = 1$ and $ad+bc=0$. From the second you get $d = -lambda b$ and $c = lambda a$ for some $lambda$. Plugging these in the first equation you end up with $lambda = frac{1}{a^2+b^2}$ and so $c = frac{a}{a^2+b^2}, d = -frac{b}{a^2+b^2}$. So your inverse is
$$beta = frac{a}{a^2+b^2}-ifrac{b}{a^2+b^2}.$$
$endgroup$
You start from $(a+bi)(c+di) = (ac-bd)+(ad+bc)i$. Then impose the conditions $ac-bd = 1$ and $ad+bc=0$. From the second you get $d = -lambda b$ and $c = lambda a$ for some $lambda$. Plugging these in the first equation you end up with $lambda = frac{1}{a^2+b^2}$ and so $c = frac{a}{a^2+b^2}, d = -frac{b}{a^2+b^2}$. So your inverse is
$$beta = frac{a}{a^2+b^2}-ifrac{b}{a^2+b^2}.$$
answered Apr 14 '18 at 9:42
GibbsGibbs
5,4103827
5,4103827
add a comment |
add a comment |
$begingroup$
You have the following:
$ac-bd=1$, Eq. 1
$bc+ad=0$, Eq. 2
Recognize this as a linear system of two equations in two unknowns $c, d$ and solve using linear combinations. But before you do so check the matrix of known coefficients:
$left[begin{matrix}a&-b\b&aend{matrix}right]$
Observe that this guarantees a unique solution if and only if the determinant $a^2+b^2$ is nonzero, but for real numbers $a$ and $b$ this requirement is met unless $a=b=0$; the square of one nonzero number is (strictly) positive and the square of the other real number is never negative. Thereby, a unique solution is guaranteed for any nonzero complex number input $a+bi$.
Now for a linear combination solution. Setting the multipliers to make the terms containing $d$ cancel gives:
$a^2c-abd=a$, Eq. 1 × $a$
$b^2c+abd=0$, Eq. 2 × $b$
And then the sum contains only one unknown:
$(a^2+b^2)c=a, c=frac{a}{a^2+b^2}$
Substitute this for $c$ in either Eq. 1 or Eq. 2 and you can then solve for $d=-frac{b}{a^2+b^2}$.
$endgroup$
add a comment |
$begingroup$
You have the following:
$ac-bd=1$, Eq. 1
$bc+ad=0$, Eq. 2
Recognize this as a linear system of two equations in two unknowns $c, d$ and solve using linear combinations. But before you do so check the matrix of known coefficients:
$left[begin{matrix}a&-b\b&aend{matrix}right]$
Observe that this guarantees a unique solution if and only if the determinant $a^2+b^2$ is nonzero, but for real numbers $a$ and $b$ this requirement is met unless $a=b=0$; the square of one nonzero number is (strictly) positive and the square of the other real number is never negative. Thereby, a unique solution is guaranteed for any nonzero complex number input $a+bi$.
Now for a linear combination solution. Setting the multipliers to make the terms containing $d$ cancel gives:
$a^2c-abd=a$, Eq. 1 × $a$
$b^2c+abd=0$, Eq. 2 × $b$
And then the sum contains only one unknown:
$(a^2+b^2)c=a, c=frac{a}{a^2+b^2}$
Substitute this for $c$ in either Eq. 1 or Eq. 2 and you can then solve for $d=-frac{b}{a^2+b^2}$.
$endgroup$
add a comment |
$begingroup$
You have the following:
$ac-bd=1$, Eq. 1
$bc+ad=0$, Eq. 2
Recognize this as a linear system of two equations in two unknowns $c, d$ and solve using linear combinations. But before you do so check the matrix of known coefficients:
$left[begin{matrix}a&-b\b&aend{matrix}right]$
Observe that this guarantees a unique solution if and only if the determinant $a^2+b^2$ is nonzero, but for real numbers $a$ and $b$ this requirement is met unless $a=b=0$; the square of one nonzero number is (strictly) positive and the square of the other real number is never negative. Thereby, a unique solution is guaranteed for any nonzero complex number input $a+bi$.
Now for a linear combination solution. Setting the multipliers to make the terms containing $d$ cancel gives:
$a^2c-abd=a$, Eq. 1 × $a$
$b^2c+abd=0$, Eq. 2 × $b$
And then the sum contains only one unknown:
$(a^2+b^2)c=a, c=frac{a}{a^2+b^2}$
Substitute this for $c$ in either Eq. 1 or Eq. 2 and you can then solve for $d=-frac{b}{a^2+b^2}$.
$endgroup$
You have the following:
$ac-bd=1$, Eq. 1
$bc+ad=0$, Eq. 2
Recognize this as a linear system of two equations in two unknowns $c, d$ and solve using linear combinations. But before you do so check the matrix of known coefficients:
$left[begin{matrix}a&-b\b&aend{matrix}right]$
Observe that this guarantees a unique solution if and only if the determinant $a^2+b^2$ is nonzero, but for real numbers $a$ and $b$ this requirement is met unless $a=b=0$; the square of one nonzero number is (strictly) positive and the square of the other real number is never negative. Thereby, a unique solution is guaranteed for any nonzero complex number input $a+bi$.
Now for a linear combination solution. Setting the multipliers to make the terms containing $d$ cancel gives:
$a^2c-abd=a$, Eq. 1 × $a$
$b^2c+abd=0$, Eq. 2 × $b$
And then the sum contains only one unknown:
$(a^2+b^2)c=a, c=frac{a}{a^2+b^2}$
Substitute this for $c$ in either Eq. 1 or Eq. 2 and you can then solve for $d=-frac{b}{a^2+b^2}$.
edited Apr 14 '18 at 10:33
answered Apr 14 '18 at 10:12
Oscar LanziOscar Lanzi
13.2k12136
13.2k12136
add a comment |
add a comment |
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