Electronic Journal of Differential Equations, Vol. 2013 (2013), No. 255, pp. 1-6. Title: Lavrent'ev problem for separated flows with an external perturbation Authors: Dmitriy K. Potapov (Saint Petersburg State Univ, Russia) Victoria V. Yevstafyeva (Saint Petersburg State Univ, Russia) Abstract: We study the Lavrent'ev mathematical model for separated flows with an external perturbation. This model consists of a differential equation with discontinuous nonlinearity and a boundary condition. Using a variational method, we show the existence of a semiregular solution. As a particular case, we study the one-dimensional model. Submitted July 2, 2013. Published November 20, 2013. Math Subject Classifications: 35J25, 35J60, 35Q35. Key Words: Lavrent'ev model; separated flows; external perturbation; discontinuous nonlinearity; semiregular solution; variational method.