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Beltrami identity
Special case of the Euler-Lagrange equations From Wikipedia, the free encyclopedia
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The Beltrami identity, named after Eugenio Beltrami, is a special case of the Euler–Lagrange equation in the calculus of variations.
The Euler–Lagrange equation serves to extremize action functionals of the form
where and are constants and .[1]
If , then the Euler–Lagrange equation reduces to the Beltrami identity,
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Derivation
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By the chain rule, the derivative of L is
Because , we write
We have an expression for from the Euler–Lagrange equation,
that we can substitute in the above expression for to obtain
By the product rule, the right side is equivalent to
By integrating both sides and putting both terms on one side, we get the Beltrami identity,
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Applications
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Solution to the brachistochrone problem

An example of an application of the Beltrami identity is the brachistochrone problem, which involves finding the curve that minimizes the integral
The integrand
does not depend explicitly on the variable of integration , so the Beltrami identity applies,
Substituting for and simplifying,
which can be solved with the result put in the form of parametric equations
with being half the above constant, , and being a variable. These are the parametric equations for a cycloid.[3]
Solution to the catenary problem
Consider a string with uniform density of length suspended from two points of equal height and at distance . By the formula for arc length, where is the path of the string, and and are the boundary conditions.
The curve has to minimize its potential energy and is subject to the constraint where is the force of gravity.
Because the independent variable does not appear in the integrand, the Beltrami identity may be used to express the path of the string as a separable first order differential equation
where is the Lagrange multiplier.
It is possible to simplify the differential equation as such:
Solving this equation gives the hyperbolic cosine, where is a second constant obtained from integration
The three unknowns , , and can be solved for using the constraints for the string's endpoints and arc length , though a closed-form solution is often very difficult to obtain.
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Notes
- Thus, the Legendre transform of the Lagrangian, the Hamiltonian, is constant along the dynamical path.
References
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