  This is a historical note. Bethe Ansatz solvable models are considered, for
example XXZ Heisenberg anti-ferromagnet and Bose gas with delta interaction.
Periodic boundary conditions lead to Bethe equation. The square of the norm of
Bethe wave function is equal to a determinant of linearized system of Bethe
equations (determinant of matrix of second derivatives of Yang action). The
proof was first published in Communications in Mathematical Physics, vol 86,
page 391 in l982. Also domain wall boundary conditions for 6 vertex model were
discovered in the same paper [see Appendix D]. These play an important role for
algebraic combinatorics: alternating sign matrices, domino tiling and plane
partition. Many publications are devoted to six vertex model with domain wall
boundary conditions.
