“…To deduce the characteristic equation, which corresponds to Equation (29) and determines nonlocal symmetry of the Equation (4) in terms of variables x, t, u(x, t), we need to exclude a variable v(x, t) from expression Equation (30), where b (v, t) is a function belonging to the set of solutions of the linear heat Equation (21). New designations will be necessary for us in what follows.…”