MathOverflow

Sometimes, people write a theorem and in the proof section,instead of actually writing the proof, people write "Trivial" or "Obvious" or "Left as an exercise to the reader". I have read some interesting like once I saw "Proof : Ask a toddler on the street...

When is it possible to explicitly compute the action of the Frobenius on the $\ell$-adic cohomology (without presupposing the truth of the Weil Conjectures, the Lefschetz formula, or importing any outside concepts) for a family of varieties of some kind...

We have a long message $m$ to encode. The message is composed of a set of symbols $\{s_i\}$. Let $p_i$ denote the number of appearance of $s_i$ in $m$. We seek to find a prefix-free code for each $s_i$ so as to minimize $\frac{N_0N_1}{N}$, where $N_...

EDIT: Let $\Omega\subset \mathbb{R}^3$ be a bounded domain with smooth connected boundary. Let $f\colon \mathbb{R}^3\backslash \Omega \to \mathbb{R}$ be a continuous function which is harmonic in $\mathbb{R}^3\backslash \bar\Omega$ and constant on $...