Erdős–Selfridge odd covering problem

A covering system of the integers is a finite family of congruence classes ai(modni)a_i \pmod{n_i} whose union is all of Z\mathbb{Z}. The moduli are odd, distinct, and greater than 11 when every nin_i is odd, the moduli are pairwise distinct, and ni>1n_i>1 for every ii.

Erdős–Selfridge odd covering problem. Does there exist a covering system of Z\mathbb{Z} whose moduli are odd, distinct, and greater than 11?

The problem is open in both directions and is Erdős's problem #7. An odd covering system would have consequences for the reducibility of trinomials, and the paper gives a kernel-checked exclusion showing that any such covering would have least common multiple exceeding 1000010000.

Sources & referencesView supporting material

Primary source

Ibrahim Mian and Shayaan Siddique, “Kernel-Checked Exclusions for the Erdős-Selfridge Odd Covering Problem: Any Odd Covering of Z Has lcm Exceeding 10000”, arXiv:2607.25628 (2026).

Additional references

3 papers in this index state this conjecture (2018–2026). The statement above is taken from the most recent of them; the others are arXiv:2507.23041, arXiv:1811.03547.

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