Is $\mathrm{lcm}(1,\ldots,p_{k+1}-1) < p_k\cdot\mathrm{lcm}(1,\ldots,p_k)$ for every $k$? (Erdős #458)
Statement
Let $[1,\ldots,n]$ denote the least common multiple of $\{1,\ldots,n\}$, and let $p_k$ denote the $k$-th prime. Is it true that, for all $k\geq 1$, $$[1,\ldots,p_{k+1}-1]< p_k\,[1,\ldots,p_k]\,?$$ Since $\mathrm{lcm}(1,\ldots,n)$ increases exactly at prime powers, the ratio $[1,\ldots,p_{k+1}-1]/[1,\ldots,p_k]$ equals the product of $q$ over all prime powers $q^j$ (with $j\geq 2$, $q$ prime) lying in the interval $(p_k,\,p_{k+1}-1]$ — so the question is whether that product of new prime-power contributions in a prime gap can ever reach $p_k$. A single explicit $k$ violating the inequality would disprove it.
Acceptance. FULLY RESOLVES (disproof): an explicit $k$ with the consecutive primes $p_k,p_{k+1}$ (primality certificates for both, compositeness witnesses for everything between), the complete list of prime powers $q^j\in(p_k,p_{k+1}-1]$ with $j\geq 2$, and a verification that the product of their bases $q$ is $\geq p_k$ — this certifies $[1,\ldots,p_{k+1}-1]\geq p_k[1,\ldots,p_k]$ without computing the lcms, and is fully machine-checkable. FULLY RESOLVES (proof): a proof of the inequality for all $k\geq 1$; machine-checkable (Lean — a formal statement already exists in google-deepmind/formal-conjectures) preferred, else a complete written proof (note it must inherently control prime gaps at Legendre strength; a computation alone cannot close it). ADVANCES: (a) a certified exhaustive verification for all $k$ with $p_k\leq N$, for an $N$ stated by the submission and strictly beyond any previously published verification it can identify (the source page records none), delivered via the prime-power-in-gaps reduction with code and an exhaustiveness argument; or (b) a conditional proof under an explicitly stated prime-gap hypothesis (e.g. $p_{k+1}-p_k<p_k^{1/2}$), handling the small-prime-power obstruction unconditionally, clearly flagged as conditional. Deliver the counterexample certificate, or the verification code + attained bound, or the proof file.
Background
A problem of Erdős and Graham [ErGr80, p.91], who write that the inequality is 'almost certainly' true but that a proof is out of reach; listed as open on erdosproblems.com/458 (fetched 2026-07-13, status 'falsifiable' — open, but a finite counterexample would disprove it). Two obstructions to a proof are recorded. First, one must rule out two primes $q$ with $p_k<q^2<p_{k+1}$: two squares of primes in one prime gap would contribute a factor $q_1q_2>p_k$ and break the inequality; excluding this needs $p_{k+1}-p_k<p_k^{1/2}$, essentially Legendre's conjecture — far stronger than anything provable today, even on RH. Second, higher powers of small primes landing in the gap also cause trouble (already at $k=4$: the gap $(7,11)$ contains $8=2^3$ and $9=3^2$, giving ratio $6$ against the allowed $<7$). The relevant lcm values $[1,\ldots,p_k]$ are OEIS A056604. No exhaustive verification bound is recorded on the page. (The venue separately hosts Erdős #291, on the numerator of the harmonic sum versus $\mathrm{lcm}(1,\ldots,n)$ — the same central object but an unrelated question.) The attacker's tools: the prime-power reduction makes large-scale verification cheap — enumerate all prime powers $q^j\leq N$ ($j\geq 2$; only $O(\sqrt{N})$ of them), locate the consecutive-prime gap containing each, and check per-gap products against $p_k$, so a workstation can certify the conjecture to very large $N$ without ever computing the astronomically large lcms; on the theory side, a conditional proof under stated prime-gap hypotheses that also tames the small-prime powers would isolate exactly what is missing.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #458 (T. F. Bloom) | website |
| REF-02 | OEIS A056604 — a(n) = lcm(1, 2, 3, ..., prime(n)) | website |
| REF-03 | Legendre's conjecture (Wikipedia) — the gap bound the proof would need | website |
| REF-04 | Formal Lean statement of Erdős #458 (google-deepmind/formal-conjectures) | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.