Open-problem statements, with their sources attached.

Szabo asks whether the maximal $t$ is given by

Is there a covering system all of whose moduli are of the form $p-1$ for some primes $p \geq 5$?

Let $A\subseteq \mathbb{N}$ be an infinite set and consider the following greedy algorithm for a rational $x\in (0,1)$: choose the minimal $n\in A$ such that $n\geq 1/x$ and repeat with $x$ replaced by $x-\frac{1}{n}$. If this terminates after finitely many steps then this produces a representation of $x$ as the sum of distinct unit fractions with denominators from $A$.

Does this process always terminate if $x$ has odd denominator and $A$ is the set of odd numbers?

More generally, for which pairs $x$ and $A$ does this process terminate?

Graham has shown that $\frac{m}{n}$ is the sum of distinct unit fractions with denominators $\equiv a\pmod{d}$ if and only if $\left(\frac{n}{(n,a,d)},\frac{d}{(a,d)}\right)=1.$ Does the greedy algorithm always terminate in such cases?

Graham has also shown that $x$ is the sum of distinct unit fractions with square denominators if and only if $x\in [0,\pi^2/6-1)\cup [1,\pi^2/6)$. Does the greedy algorithm for this always terminate? Erdős and Graham believe not - indeed, perhaps it fails to terminate almost always.

Let $k(N)$ denote the smallest $k$ such that there exists $N ≤ n_1 < ⋯ < n_k$ with $\frac 1 {n_1} + ... + \frac 1 {n_k} = 1$

Is it true that $\lim_{N \to \infty} k(N) - (e - 1)N = \infty$?

There are no examples known of the weakened coprime version if we insist that $1\not\in P\cup Q$.

Does there exist a constant c > 0 such that, for any K > 1, whenever A is a sufficiently large finite multiset of integers with $\sum_{n \in A} 1/n > K$ there exists some $S \subseteq A$ such that $1 - \exp(-(c*K)) < \sum_{n \in S} 1/n \le 1$?

What is the size of the largest $A\subseteq\{1, \dots, N\}$ such that there is a function $\delta : A \to \{-1, 1\}$ such that

and

for all non-empty $A'\subsetneq A$.

Let $c(N)$ be the size of the largest $A\subseteq\{1, \dots, N\}$ such that there is a function $\delta : A \to \{-1, 1\}$ such that

and

for all non-empty $A'\subsetneq A$. What is $\Theta(c(N))$?

Let $c(N)$ be the size of the largest $A\subseteq\{1, \dots, N\}$ such that there is a function $\delta : A \to \{-1, 1\}$ such that

and

for all non-empty $A'\subsetneq A$. Find the simplest $g(N)$ such that $c(N) = O(g(N)).

Let $c(N)$ be the size of the largest $A\subseteq\{1, \dots, N\}$ such that there is a function $\delta : A \to \{-1, 1\}$ such that

and

for all non-empty $A'\subsetneq A$. Find the simplest $g(N)$ such that $c(N) = o(g(N)).

Let $A \subset \mathbb{N}$ be an additive basis of order 2.

Must there exist $B = \{b_1 < b_2 < \dots\} \subseteq A$ which is also a basis such that $\lim_{k\to\infty} \frac{b_k}{k^2}$ does not exist?

Erdős Problem 329.* Let A ⊆ ℕ be a Sidon set. How large can lim sup_{N → ∞} |A ∩ {1,…,N}| / N^{1/2} be?

The converse: if the maximum density is 1, then any finite Sidon set can be embedded in a perfect difference set modulo $n > 0$.

Since the consequent is false (due to the counterexamples in [Ha47] and [AlMi25]), this implication is logically equivalent to the statement that the maximum upper density of Sidon sets is NOT 1. Because the maximum upper density problem is still open, the truth value of this implication is also an open research problem.

Does there exist a minimal basis $A \subset \mathbb{N}$ with positive density such that, for any $n \in A$, the (upper) density of integers which cannot be represented without using $n$ is positive?

Let $A = \{1, 2, 4, 8, 13, 21, 31, 45, 66, 81, 97, \ldots\}$ be the greedy Sidon sequence: we begin with $1$ and iteratively include the next smallest integer that preserves the Sidon property (i.e. there are no non-trivial solutions to $a + b = c + d$). What is the order of growth of $A$? Is it true that $|A \cap \{1, \ldots, N\}| \gg N^{1/2 - \varepsilon}$ for all $\varepsilon > 0$ and large $N$?

Let $A = \{1, 2, 4, 8, 13, 21, 31, 45, 66, 81, 97, \ldots\}$ be the greedy Sidon sequence: we begin with $1$ and iteratively include the next smallest integer that preserves the Sidon property (i.e. there are no non-trivial solutions to $a + b = c + d$). What is the order of growth of $A$? Is it true that $|A \cap \{1, \ldots, N\}| \gg N^{1/2 - \varepsilon}$ for all $\varepsilon > 0$ and large $N$?

Erdős and Graham [ErGr80] also asked about the difference set $A - A$ and whether this has positive density.

[ErGr80] Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory. Monographies de L'Enseignement Mathematique (1980).