“We develop a {{w|Scientific formalism|formalism}} for computing sums over random surfaces which arise in all problems containing {{w|gauge invariance}} (like {{w|Quantum chromodynamics|QCD}}, three-dimensional {{w|Ising model}} etc.). These sums are reduced to the exactly solvable quantum theory of the two-dimensional Liouville lagrangian. At D = 26 the string dynamics is that of harmonic oscillators as was predicted earlier by dual theorists, otherwise it is described by the nonlinear integrable theory.”