  We introduce high-energy limits which allow us to derive recursion relations
fixing the various couplings of Lagrangians of two-dimensional relativistic
quantum field theories with no tree-level particle production in a very
straightforward way. The sine-Gordon model, the Bullough-Dodd theory, Toda
theories of various kinds and the U(N) non-linear sigma model can all be
rediscovered in this way. The results here were the outcome of our explorations
at the 2017 Perimeter Institute Winter School.
