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 classification, 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.