MathOverflow

A continuous field of $C^∗$-algebras $(A_x)_{x∈X}$ where $X$ is a topological space, is a $∗$-subalgebra $Γ⊆ℓ^∞\text{-}∏_{x∈X}A_x$ such that
$\{a_x:a∈Γ\}⊆A_x$ is dense for each $x∈X$,
$x↦\|a_x\|$ is continuous for all $a∈Γ$,

To continue with the topic of Plane sextics admitting a conic with 6 double-contact points, I would like to understand the variety of conics touching a given (irreducible) projective plane sextic curve $\{P=0\}$ in 6 points (a.k.a. double contact conics...

Is there a way to efficiently check if all matrices in the following set are Hurwitz stable (eigenvalues strictly in the left-hand plane)?$$\left\{ A \in \Bbb R^{n \times n} : \ell_{i,j}\leq A_{i,j} \leq u_{i,j} \right\}$$ I could work with eigenvalues...

I am not sure I can strictly define recreational mathematics. But we all feel what it is about: puzzles, problems you can ask your mathematical friends, problems that will bother them for a couple of hours, and then they will get them (or not?). Problems...