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Questions on the calculus of variations, which deals with the optimization of functionals mostly defined on infinite dimensional spaces.

1 vote

Singularity at left endpoint for variational calculus problem

The answer is yes, if the interval you mean is $(0,\pi/2)$. That is, in any case, the natural domain on which the Euler-Lagrange equation $EL(x)$ is expected to hold. The reason is that it reflects th …
Igor Khavkine's user avatar
3 votes

Conditions ensuring extrema are twice continuously differentiable?

I believe that a piecewise smooth extremum would have to satisfy the Weierstrass-Erdmann corner conditions. If these conditions ensure that the extremum is in fact $C^1$, then it solves the Euler-Lagr …
Igor Khavkine's user avatar
1 vote

Functional Minimization: When is this heuristic rigorous?

A useful and fairly complete reference on this and related questions is Morrey Multiple Integrals in the Calculus of Variations. http://books.google.com/books?id=-QNKm1PBohsC
Igor Khavkine's user avatar
5 votes
Accepted

Euler operator as part of a cochain complex

Yes. The next operator in the sequence is called the Helmholtz operator, followed by higher versions thereof. The main keyword is "variational bicomplex" and a standard reference is I. M. Anderson, “ …
Igor Khavkine's user avatar
1 vote

Variational principle for relativistic gas dynamics

For now I'll just mention that there's a small literature on variational principles for perfect fluids in relativity, though I'm not an expert on it. Here is a reference that discusses some approaches …
Igor Khavkine's user avatar
3 votes
Accepted

Extending the variational bicomplex to Hamiltion or Hamiltion-Jacobi formalism

I'm not sure that there is a right answer to your question, since what is and what is not a satisfactory generalization of symplectic geometry can be quite subjective and in the end only can judge wha …
Igor Khavkine's user avatar
4 votes

In which sense are Euler-Lagrange PDE's on fiber bundles quasi-linear?

Let $\mathcal{E} \subset J^{2k}E$ be the submanifold (provided that this subset is a submanifold) of all $2k$-jets sitting in the zero-level set of $\rho(E(\mathscr{L}))$, the PDE submanifold. This is …
Igor Khavkine's user avatar
0 votes

How to solve an optimization problem whose optimization variable is a function?

Differentiating the left-hand side of the second condition I get $b^{-1}(f(x) + \int_x^\infty f(t) dt)$, not just $b^{-1} f(x)$, but it is still $\ge 0$. So the second condition can be replaced by its …
Igor Khavkine's user avatar
0 votes

Integral identity for critical points of the Ginzburg-Landau functional

If I'm not miscalculating, the variational equation for $\varphi$ gives $\delta E_\epsilon/\delta\varphi = \partial^k (\rho^2 \partial_k \varphi) = 0$. Writing your integral identity as a flux through …
Igor Khavkine's user avatar
2 votes

Degenerate second-order Lagrangians

I think you might find interesting §4.B of Anderson's The Variational Bicomplex [1] (Theorems 4.23, 4.29 and Corollary 4.30, to be more precise). However, these results are in a sense converse to your …
Igor Khavkine's user avatar
11 votes

Why the least action principle is always (?) used in this particular form?

In the form (1), if you compute the variation $\delta S / \delta x(t) = E(t)$, you find that $E(t) = E(x(t),\dot{x}(t), \ddot{x}(t) ,t)$ is a local/differential expression (the value of $E(t)$ does no …
Igor Khavkine's user avatar
3 votes
Accepted

Different smooth structures on the infinite jet bundle (for the purposes of calculus of vari...

The following remarks are based on having previously gone through the literature that you've mentioned also for the purposes of figuring out these differences. It has been a while since then, but the …
Igor Khavkine's user avatar
5 votes

Classification of Lagrangians with given Euler-Lagrange equations

In a sense, all the Lagrangians giving the same Euler-Lagrange equations are exhausted by transformations of your type (b), which adds a total derivative/total divergence/boundary term/... Transforma …
Igor Khavkine's user avatar