The well-known steady-state problem of impingement of two plane jets of an ideal incompressible fluid moving with the same velocity is refined and extended to the case of unsteady interaction. Equations describing perturbation propagation on the free surface of the impinging jets are obtained and linearized on the steady-state solution.Introduction. The problem of impingement of two jets moving at an angle toward each other with the same velocity which can be considered equal to unity without loss of generality is one of the simplest problems solved by the Kirchhoff method [1]. It arises from the problem of fluid flow from two hoses (more precisely, slots) of different thicknesses (Fig. 1a) in the limit where these hoses move to infinity in directions opposite to the directions of the jet velocities. This results in a steady-state jet flow which is shown schematically in Fig. 1b.Investigation of unsteady flow is necessary, for example, for impingement of jets moving at different velocities. In this case, a continuous steady-state solution cannot be constructed because different Bernoulli constants cannot provide pressure continuity at the critical point. A moving coordinate system in which the flow is steadystate can be found only in the particular case of head-on impingement where the jet velocities are different and directed toward each. In the case where the jets impinge at an angle, such a system does not exist. Unlike in the case of head-on impingement, in this case there is a distinguished point, for example, the point of intersection of the midline of the impingement jets (point A in Fig. 1a). In a coordinate system rigidly attached to this point, the flow is apparently periodic in time, which is confirmed by experimental studies [2] which showed the existence of aperiodic waves on the free surface during interaction of two oil flows moving at different velocities. This periodic interaction of jets can be regarded as the simplest model of wave formation during explosion welding [3]. More complex models taking into account acoustic perturbations, viscosity, thermal conductivity, surface tension, instability of the von Kármán vortex street, etc., are presented in [4].The equations obtained below can be used to describe the interaction of jets moving at different velocities. However, first of all, it is necessary to derive equations to study the stability of steady-state impingement of jets moving at the same velocity. In this case, it is assumed that perturbations decrease at infinity, i.e., do not influence the steady-state flow velocity at points at infinity.Slightly perturbed jet flows are commonly studied by using the equations given in [5] with the boundary conditions extended to the unperturbed free surface. The question of the validity of this procedure for the case of unsteady interaction of jets where there are four points at infinity remains open. In this case, in fact, a conformal mapping of the perturbed flow region onto the unperturbed region is performed. It is not clear whether it is possible ...