Symmetries and Applications of Differential Equations : In Memory of Nail H. Ibragimov (1939–2018) Approximate symmetry in nonlinear physics problems One dimensional ﬂows of a polytropic gas: Lie group classiﬁcation conservation laws invariant and conservative difference schemes Hamiltonian structure and conservation laws of three dimensional linear elasticity theory Complex methods for Lie symmetry analysis Symmetry analysis and conservation laws of a family of Boussinesq equations Group Analysis of the Guéant and Pu Model of Option Pricing and Hedging On Involutive Systems of Partial Differential Equations Group analysis of some Camassa Holm type equations. 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.