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I am a professor of Mathematics at the University of Pisa, Italy.
22h

awarded  Popular Question 
May 18 
awarded  Nice Question 
May 14 
awarded  Yearling 
May 6 
comment 
Orthogonal complements of intersections of closed subspaces
Yes, it's easy that $(K_1+K_2)^\perp =K_1^\perp \cap K_2^\perp$ holds for linear subspaces, and you can generalize by induction to n subspaces. Then if you take another orthogonal, you get closures (and you can also rename $H_j:=K_j^\perp$) 
May 5 
comment 
Orthogonal complements of intersections of closed subspaces
In general it is not true even for $n=2$, because $(H_1\cap H_2)^{\perp}$ is always closed, while the sum of two closed subspaces may fail to be closed. (For instance, in $H:=X\times X$ take the graph of the zero operator as $H_1^{\perp}$ and the graph of a dense, nonsurjective bounded operator $T$ on $X$ as $H_2^{\perp}$: then $H_1^{\perp} + H_2^{\perp}= X\times T(X)$ is a dense not closed subspace). 
Apr 29 
comment 
Which popular games are the most mathematical?
The dual question is also intriguing: which proofs of mathematical theorems have a structure that resemble the most a strategy of a game? 
Apr 27 
comment 
Invariant mesures for expanding maps of the circle
ah, ok thanks I assumed T was a diffeo :) 
Apr 27 
comment 
Best Hölder exponents of surjective maps from the unit square to the unit cube
yes, thank you; fixed. 
Apr 27 
revised 
Best Hölder exponents of surjective maps from the unit square to the unit cube
edited body 
Apr 27 
revised 
Best Hölder exponents of surjective maps from the unit square to the unit cube
added 138 characters in body 
Apr 27 
revised 
Best Hölder exponents of surjective maps from the unit square to the unit cube
edited title 
Apr 27 
revised 
Best Hölder exponents of surjective maps from the unit square to the unit cube
edited title 
Apr 27 
asked  Best Hölder exponents of surjective maps from the unit square to the unit cube 
Apr 26 
comment 
For a nonconvex function f, how to find a function g such that $g\circ f$ is strictly convex?
Then, if g is increasing and g(f(x) is convex, all sublevel sets of f, {x : f(x)<c} = {x : g(f(x)) < g(c)} must be intervals. 
Apr 26 
comment 
For a nonconvex function f, how to find a function g such that $g\circ f$ is strictly convex?
What assumptions are you making on $f$, and what are you requiring from $g$ ? Note that e.g. for $f(x):=\sin(x)$, the function $g(\sin(x))$ is constant on $\pi\mathbb{Z}$, so it is convex only if it is constant. 
Apr 25 
comment 
Two questions on hyperspace of a metric space
As to the first question, you may also observe that if $(X,d)$ is a compact metric space, its hyperspace $H(X)$ (the set of closed sets of $X$ endowed with the Hausdorff distance) is a compact metric space too; moreover the construction extends to a functor from the category of compact metric spaces & continuous functions into itself, precisely : if $f:X\to Y$ is continuous the map $K\to f(K)$ is continuous from $H(X)$ to $H(Y)$ (just because $f$ is uniformly continuous). In particular, if $(X,d)$ and $(X',d')$ are homeomorphic compact metric spaces, so are their hyperspaces. 
Apr 23 
awarded  Good Answer 
Apr 22 
revised 
Functional minimization problem
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Apr 21 
answered  Functional minimization problem 
Apr 18 
revised 
Biggest parallelogram inside the union of two translated parallelograms
added 81 characters in body 