An explicit Steiner system with t ≥ 6

Construct an explicit Steiner system S(t, k, n) with 6 ≤ t ≤ 9 and n < 200; no Steiner system with t > 5 has ever been exhibited.

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State the problem, from FrontierMath: Open Problems

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	1	+# An explicit Steiner system with t ≥ 6
	2	+
	3	+A Steiner system $S(t, k, n)$ is a collection of $k$-element blocks from $n$ points such that every $t$ points lie in exactly one block. Keevash proved in 2014 that such systems exist for every $t$ whenever $n$ is large and the obvious divisibility conditions hold, yet no example with $t \ge 6$ has ever been written down. The task is to find one of modest size.
	4	+
	5	+## Problem
	6	+
	7	+An $S(t, k, n)$ Steiner system is a set $\mathcal{B}$ of $k$-element subsets ("blocks") of $\{1, \dots, n\}$ such that every $t$-element subset of $\{1, \dots, n\}$ is contained in exactly one block.
	8	+
	9	+Task. Construct an explicit $S(t, k, n)$ with
	10	+$$n > k > t \ge 6, \qquad t \le 9, \qquad n < 200.$$
	11	+
	12	+(Epoch writes these as $(n, q, r)$-systems with $q = k$ and $r = t$.)
	13	+
	14	+## Background
	15	+
	16	+Questions about such designs go back to Plücker, Kirkman and Steiner in the nineteenth century, and Keevash's theorem answered Steiner's 1853 question about their existence [1]. His proof, and the later ones by Glock, Kühn, Lo and Osthus [2], by Delcourt and Postle [3] and by Keevash again [4], are probabilistic or absorption-based. They guarantee existence only for enormous $n$ and give no usable example. Explicit Steiner systems are known only for $t \le 5$, for instance the Witt designs $S(5, 6, 12)$ and $S(5, 8, 24)$. Explicit designs matter in practice, for experimental design and error-correcting codes.
	17	+
	18	+## What is known
	19	+
	20	+- For all fixed $t < k$, $S(t, k, n)$ exists for all sufficiently large $n$ satisfying the divisibility conditions [1–4].
	21	+- No Steiner system with $t \ge 6$ has been exhibited (Epoch; also recorded in formal-conjectures).
	22	+- Epoch's problem author doubts that even an optimised brute-force search could succeed, and expects a new conceptual approach to be needed.
	23	+
	24	+## What would count as progress
	25	+
	26	+- The parameters and the list of blocks. Checking is direct: either count, for each $t$-subset, the blocks containing it, or check that any two blocks share fewer than $t$ points and that there are exactly $\binom{n}{t} / \binom{k}{t}$ blocks.
	27	+- Epoch scores submissions with its own verifier, given $n$, $q = k$, $r = t$ and the blocks.
	28	+- formal-conjectures states exactly this task in Lean as `large_steiner_systems` in [SteinerSystem.lean](https://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/Wikipedia/SteinerSystem.lean).
	29	+- A construction with a large symmetry group, which explains why it works, would likely point to further examples.
	30	+
	31	+## References
	32	+
	33	+1. P. Keevash, The existence of designs, 2014. [arXiv:1401.3665](https://arxiv.org/abs/1401.3665)
	34	+2. S. Glock, D. Kühn, A. Lo and D. Osthus, The existence of designs via iterative absorption: hypergraph $F$-designs for arbitrary $F$, 2016. [arXiv:1611.06827](https://arxiv.org/abs/1611.06827)
	35	+3. M. Delcourt and L. Postle, Refined absorption: a new proof of the existence conjecture, 2024. [arXiv:2402.17855](https://arxiv.org/abs/2402.17855)
	36	+4. P. Keevash, A short proof of the existence of designs, 2024. [arXiv:2411.18291](https://arxiv.org/abs/2411.18291)
	37	+
	38	+## Source
	39	+
	40	+Imported from [FrontierMath: Open Problems, large-steiner-systems](https://epoch.ai/frontiermath/open-problems/large-steiner-systems), curated by Epoch AI. Open according to the source as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.