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In 1973 the Chinese mathematician J.-R. Chen proved that there are infinitely many primes $p$ such that $p+2$ is a product of at most two primes. Nowadays such primes $p$ are called Chen primes. For $...

In Question 315259 (cf. Primes arising from permutations) I asked a question on primes arising from permutations which looks quite challenging. Here I pose a new question in this direction which does ...

For a permutation $\sigma\in S_n$, its crossing number $\text{cr}(\sigma)$ is the number of pairs $\{i,j\}$ with $i,j\in\{1,\ldots,n\}$ such that $$i<j\le\sigma(i)<\sigma(j)\ \ \text{or}\ \ \...