AKNS Lax pair for the nonlinear Schrodinger equation (source code)

= AKNS Lax pair for the nonlinear Schrodinger equation
{c}

For complex fields $q,r$, the standard two-by-two AKNS pair has zero-curvature equations
$$
ir_t+r_{xx}+2qr^2=0,
\qquad
iq_t-q_{xx}-2rq^2=0.
$$
The reductions $q=\overline r$ and $q=-\overline r$ give the focusing and defocusing cubic nonlinear Schrödinger equations, respectively.