Book contents
- Frontmatter
- Contents
- Preface to the second edition
- Preface to the first edition
- 1 Preliminary algebra
- 2 Preliminary calculus
- 3 Complex numbers and hyperbolic functions
- 4 Series and limits
- 5 Partial differentiation
- 6 Multiple integrals
- 7 Vector algebra
- 8 Matrices and vector spaces
- 9 Normal modes
- 10 Vector calculus
- 11 Line, surface and volume integrals
- 12 Fourier series
- 13 Integral transforms
- 14 First-order ordinary differential equations
- 15 Higher-order ordinary differential equations
- 16 Series solutions of ordinary differential equations
- 17 Eigenfunction methods for differential equations
- 18 Partial differential equations: general and particular solutions
- 19 Partial differential equations: separation of variables and other methods
- 20 Complex variables
- 21 Tensors
- 22 Calculus of variations
- 23 Integral equations
- 24 Group theory
- 25 Representation theory
- 26 Probability
- 27 Statistics
- 28 Numerical methods
- Appendix Gamma, beta and error functions
- Index
22 - Calculus of variations
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface to the second edition
- Preface to the first edition
- 1 Preliminary algebra
- 2 Preliminary calculus
- 3 Complex numbers and hyperbolic functions
- 4 Series and limits
- 5 Partial differentiation
- 6 Multiple integrals
- 7 Vector algebra
- 8 Matrices and vector spaces
- 9 Normal modes
- 10 Vector calculus
- 11 Line, surface and volume integrals
- 12 Fourier series
- 13 Integral transforms
- 14 First-order ordinary differential equations
- 15 Higher-order ordinary differential equations
- 16 Series solutions of ordinary differential equations
- 17 Eigenfunction methods for differential equations
- 18 Partial differential equations: general and particular solutions
- 19 Partial differential equations: separation of variables and other methods
- 20 Complex variables
- 21 Tensors
- 22 Calculus of variations
- 23 Integral equations
- 24 Group theory
- 25 Representation theory
- 26 Probability
- 27 Statistics
- 28 Numerical methods
- Appendix Gamma, beta and error functions
- Index
Summary
In chapters 2 and 5 we discussed how to find stationary values of functions of a single variable f(x), of several variables f(x, y, …) and of constrained variables, where x, y, … are subject to the n constraints gi(x, y, …) = 0, i = 1, 2, …, n. In all these cases the forms of the functions f and gi were known, and the problem was one of finding the appropriate values of the variables x, y etc.
We now turn to a different kind of problem in which we are interested in bringing about a particular condition for a given expression (usually maximising or minimising it) by varying the functions on which the expression depends. For instance, we might want to know in what shape a fixed length of rope should be arranged so as to enclose the largest possible area, or in what shape it will hang when suspended under gravity from two fixed points. In each case we are concerned with a general maximisation or minimisation criterion by which the function y(x) that satisfies the given problem may be found.
The calculus of variations provides a method for finding the function y(x). The problem must first be expressed in a mathematical form, and the form most commonly applicable to such problems is an integral. In each of the above questions, the quantity that has to be maximised or minimised by an appropriate choice of the function y(x) may be expressed as an integral involving y(x) and the variables describing the geometry of the situation.
- Type
- Chapter
- Information
- Mathematical Methods for Physics and EngineeringA Comprehensive Guide, pp. 834 - 861Publisher: Cambridge University PressPrint publication year: 2002