Breaking the 2ⁿ Barrier for Counting Linear Extensions with a Short Elementary Algorithm

Keigo Oka

A deterministic exact algorithm counts linear extensions of any n-element poset in O*(1.89^n) time.

Sources for this record

What was already known

The standard generic dynamic program takes O(n2^n) time. A sub-2^n algorithm had previously been known for two-dimensional posets, not arbitrary posets.

AI involvement AI-led

Oka credits Claude Opus 5 with the new first-upper-element pattern representation, multiplicity decoding, deadline dynamic program and state count. ChatGPT 5.6 Sol generated the research prompt; Oka checked the results.

Oka credits Claude Opus 5 with the pattern representation, profile decoding, deadline dynamic program and state counting. Another model generated the research prompt; Oka checked the results.

Importance Notable

The result gives a sub-2^n exact algorithm for general posets.

Limitations

The paper includes executable finite-poset verification for n≤7. That computation checks examples and parts of the construction, not the full asymptotic theorem.

Technical verification Formal (exact computation + human)

Oka says he independently checked the statements and proofs, including the asymptotic theorem. Exact computational audits compare finite cases through n=7 and sampled families; those computations do not prove the runtime bound.

Human understanding Full

Oka explains the pattern representation, decoding, deadline dynamic program and two-branch state count.

Correction and publication history