Arnold's theorem 4 Axisymmetric flow 4 Basic flow 4 Beltrami 4 Beltrami flow 4 Complex topology 4 Eigen-value 4 Eigenvalues and eigenfunctions 4 Enstrophy 4 Euler flows 4 Flow experience 4 Flow structure 4 Force-free magnetic fields 4 Gravity waves 4 Ideal fluids 4 Kelvin waves 4 Magnetic fields 4 Magnetohydrodynamics 4 Stability 4 Topology 4 Variational principles 4 Variational techniques 4