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Book: A1

See: Math Notebook, {$A_2$}, {$A_n$}

Understand {$A_1$}, a node in a Dynkin diagram.


Its commutator is one-dimensional, thus a single dot in the Dynkin diagram.

These are the rotations of a sphere. They can be represented by 2x2 complex matrices, or 1x1 quaternions.

Start with a circle group - rotations around an axis (say, z). In three-dimensional space we add rotations around two other axes (say, x and y). Then composing rotations should yield opposite effects depending on the order.


Given a matrix:

{$\begin{pmatrix} a_{11}+ib_{11} & a_{12}+ib_{12} \\ a_{21}+ib_{21} & a_{22}+ib_{22} \end{pmatrix} $}

Suppose the determinant is 1. Then the inverse is:

{$\begin{pmatrix} a_{22}+ib_{22} & -a_{12}-ib_{12} \\ -a_{21}-ib_{21} & a_{11}+ib_{11} \end{pmatrix} $}

And the conjugate transpose is:

{$\begin{pmatrix} a_{11}-ib_{11} & a_{21}-ib_{21} \\ a_{12}-ib_{12} & a_{22}-ib_{22} \end{pmatrix} $}

Setting the latter two equal, we have the equations:



Thus the matrix M is:

{$\begin{pmatrix} a_{11}+ib_{11} & -a_{12}+ib_{12} \\ a_{12}+ib_{12} & a_{11}-ib_{11} \end{pmatrix} $}

In other words, we have that M is:

{$\begin{pmatrix} a_{11}+ib_{11} & i(b_{12}-ia_{12}) \\ i(b_{12}+ia_{12}) & a_{11}-ib_{11} \end{pmatrix} $}

In terms of complex numbers, M is:

{$\begin{pmatrix} x & i\overline{y} \\ iy & \overline{x} \end{pmatrix} $}

Alternatively, M is:

{$\begin{pmatrix} x & 0 \\ 0 & \overline{x} \end{pmatrix} + i \begin{pmatrix} 0 & \overline{y} \\ y & 0 \end{pmatrix} $}

So {$i$} can be understood as signifying the permutation of the rows, which is to say, a switch of axes, a rotation of 90 degrees.


Note that we have an additional equation that the {$\textrm{det}(M)=1$}, so that {$x \overline{x} + y \overline{y} = 1$}. And {$x$} and {$y$} are complex numbers. Thus there are three independent real variables.

Parsiųstas iš http://www.ms.lt/sodas/Book/A1
Puslapis paskutinį kartą pakeistas 2020 sausio 27 d., 20:15