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For questions on divisors and multiples, mainly but not exclusively of integers, and related and derived notions such as sums of divisors, perfect numbers and so on.
6
votes
Accepted
Smallest integer not divisible by integers in a finite set
Given an integer $n$, the Jacobsthal function $g(n)$ is the least integer, so that among any $g(n)$ consecutive integers $a,a+1,\dots,a+g(n)-1$ there is at least one that is coprime to $n$. Let $\nu(n …
10
votes
Accepted
Large gaps between consecutive irreducible polynomials with small heights
At the cost of having the degree be very large you can always choose a $k$-gap with coefficients in $\lbrace 0,1\rbrace$. Pick a large $n$ so that $n\equiv -j\pmod{p_j}$, for all $1\le j\le k$. Where …
5
votes
Accepted
Trying to prove a congruence for Stirling numbers of the second kind
Here is a quick generating function argument. Start with the following lemma
Lemma: The power series $$\frac{1}{(1-x)(1-2x)\cdots (1-kx)}$$
is even $\pmod{k+1}$ when $k+1$ is odd, and even $\pmo …
4
votes
Accepted
Sum of divisors and unitary divisors as the eigenvalue and the spectral norm of some additio...
In both cases you are really only using the additive structure of your rings, so this is really a question about abelian groups.
Assuming $n = p_1^{a_1} \cdots p_r^{a_r}$, when studying $A_n$ we are w …