This book is about Lie group analysis of differential equations for physical and engineering problems. The topics include:-- Approximate symmetry in nonlinear physical problems-- Complex methods for Lie symmetry analysis-- Lie group classi?cation, Symmetry analysis, and conservation laws -- Conservative difference schemes-- Hamiltonian structure and conservation laws of three-dimensional linear elasticity-- Involutive systems of partial differential equations This collection of works is written in memory of Professor Nail H. Ibragimov (1939–2018). It could be used as a reference book in differential equations in mathematics, mechanical, and electrical engineering.