[ left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right) ]

[ left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right)
%
left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right)
]

[
begin{pmatrix}
  1 & 0 \ 0 & e^{ipi/k}
end{pmatrix}
begin{pmatrix}
  u \ v
end{pmatrix}
=
begin{pmatrix}
  u \ -v
end{pmatrix}
]

[ left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right) times left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right) =left( begin{array}{cc}
1 & 0 \
0 & 1
end{array} right)]