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

11 votes
Accepted

Functional minimization problem

Actually this is one of the oldest problems in the calculus of variations. It's named "the Newton problem" after Sir Isaac Newton, who studied it in 1685. It arises from the determination of the optim …
Pietro Majer's user avatar
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3 votes
Accepted

Critical points in Hilbert space

Especially in Calculus of Variations and Mechanics, a submanifold $Y$ of a manifold $X$ is usually called "a natural constraint" for a functional $f$ on $X$, if the special circumstance that you are c …
Pietro Majer's user avatar
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2 votes

Approximating functions in $H_0^1(\Omega)$ by piecewise affine ones.

Certainly, a piecewise affine function $f$ (meaning, a function which is affine on each open simplex of some triangulation of the domain) is in $W^{1,\\ p}_{loc}$ for some $1\leq p\leq \infty$ if and …
Pietro Majer's user avatar
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1 vote

optimize with respect to domain shape

I'd say these belongs to the family of shape optimization problems in the Calculus of Variations (of whom the most famous example is the isoperimetric problem). As to the first one, you should make s …
Pietro Majer's user avatar
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3 votes
Accepted

Is vesica piscis a maximal length curve constrained to two points?

The length of these curves is unbounded. For a positive integer $n$ consider a triangular wave $f_n:[0,1]\to\mathbb{R}$ with support on $[1/3,2/3]$, making $n$ (isosceles) triangular impulses on $[1 …
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1 vote
Accepted

Converting an integral equation into a differential equation

This is elementary but let's state it. First one only considers smooth test functions $h$ with compact support (so that the last two terms disappear). If $F(x):=\int_0^xf(t)dt$, integrating by parts t …
3 votes

Minimizing sequence $\implies$ Palais–Smale sequence

In fact it's just the MVT to $DF$: $$\|DF(x_n)-DF(y_n)\|\le \|x_n-y_n\|\sup\|D^2F\|=O(\|x_n-y_n\|)=o(1),$$ so $\|DF(x_n)\|=o(1)$ too. $$*$$ Note that if $D^2f$ is not bounded, it is not true, and (a …
Pietro Majer's user avatar
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3 votes

Choquet theory and Hilbert's fourth problem

It seems to me that the set of projective semi-distances that you describe in the example (let's call them of type I) can be slightly generalized the following way. In the definition of $d_H$, let $H …
Pietro Majer's user avatar
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