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Help with matrix form of the imaginary unit, i

  1. Jun 12, 2014 #1
    While investigating more about complex numbers today I ran across the 2x2 matrix representation of a complex number, and I was really fascinated. You can read what I read here.

    As I understand it, you write z in its binomial form but instead of "1" you use the identity matrix, I, and for i you use a matrix m such that [itex]M^2=-I[/itex], like with the more usual definition of i.

    [itex]z =x\cdot 1+y\cdot 1 \cdot i\sim z=x\cdot \begin{bmatrix}1
    &0 \\
    0 &
    1\end{bmatrix} +y\cdot \begin{bmatrix}1
    &0 \\
    0 &
    1\end{bmatrix} \begin{bmatrix}0
    &-1 \\
    1 &
    &0 \\
    0 &
    x\end{bmatrix} +\begin{bmatrix}0
    &-y \\
    y &
    &-y \\
    y &

    In the "normal" way (the one you learn first, at least in my case) i is defined as follows: [itex]i^{2}=-1[/itex], the positive solution to the equation [itex]x^2+1=0[/itex]. This equation has two solutions, and by convention i is the positive one.

    If we try to solve [itex]M^2=-I[/itex], then we get infinite possibilites:

    [itex]M^2=-I \Rightarrow -\begin{bmatrix}1
    &0 \\
    0 &1
    &b \\
    a&b \\
    &b(a+d) \\
    c(a+d)& d^2+bc
    \end{bmatrix}\Rightarrow \left\{\begin{matrix}a^2+bc=-1
    \\ d^2+bc=-1
    \\ b(a+d)=0
    \\ c(a+d)=0

    There aren't just 2 solutions now. Why is the matrix [itex]\begin{bmatrix}
    0& -1\\
    \end{bmatrix}[/itex] chosen over any of the rest of the matrices that satisfy m^2=-I? What is so convenient about that form over any other? Perhaps i isn't defined only as the solution to m^2=-I, but if so, what am I missing?

    Thank you for reading.
  2. jcsd
  3. Jun 12, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    Any matrix of the form ##U^{-1}\left[\begin{smallmatrix}0&-1\\1&0\end{smallmatrix}\right]U##, where ##U## is a unitary matrix, can be used to represent the imaginary unit.

    Mathematical groups, of which the complex numbers are one example, generally have many different representations as sets of matrices.
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