REVIEW 4 minor 36 references
Some applications of the real strict order property hierarchy
T0 review · 0 major / 4 minor · reviewed 2026-07-12 · grok-4.5
Pith's one-line read Cycle-removal from the real-valued NSOP_r hierarchy forces a sharp SOP2–SOP3 dichotomy on hereditary classes with finitely many forbidden weak embeddings.
desk verdict Solid technical progress on NSOP2 vs NSOP3 and the NTP2 collapse via real-valued SOP_r tools; the sharp hereditary-class dichotomy is the real payload. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Cycle-removal via iterated helix maps (and their abstract cycle-removal properties) on the specially constructed hereditary class H_std of sound L_std-structures that encode a non-overlapping instance of the tree property; the maps successively eliminate split cycles and then potentially pinched alternating cycles until a forbidden configuration that still embeds into the standard TP-structure is produced.
What would settle it
Exhibit a single hereditary class H defined by finitely many forbidden weakly embedded substructures such that every theory with age H has SOP2 yet some theory with age H is NSOP3 (or, equivalently, show that one of the Cherlin–Shelah–Shi theories T_H is strictly NSOP3).
Extended reading notes
Core claim
For any hereditary class H defined by a finite family of forbidden weakly embedded substructures, the universal presence of SOP2 among theories with age H already forces the universal presence of SOP3. The same implication fails if SOP2 is replaced by the tree property, so the dichotomy is sharp.
Load-bearing premise
The reduction of the dichotomy to Cherlin–Shelah–Shi generic structures rests on preservation of NSOP_n under model companions together with the existence of those companions for finitely forbidden weak embeddings; if the precise non-overlapping SOP2 configurations fail to be preserved, the combinatorial argument no longer yields the model-theoretic claim.
Editorial extensions
If this is right
- If every well-defined NSOP_r for non-integer r is new, then NSOP2 is strictly weaker than NSOP3.
- If the real- and integer-valued hierarchies are non-distinct on the stated general combinatorial grounds, then NSOP_n ∩ NTP2 = NSOP_{n+1} ∩ NTP2 for every n ≥ 3.
- Any directed graph definable in an NTP2 theory that omits some finite digraph must omit arbitrarily large cycle-free digraphs.
- Approximate implications NSOP3 ⇝ NSOP2 and NSOP3 ⇝ NTP2 hold in the sense that formulas satisfying NSOP3 look arbitrarily close to forbidding the corresponding configurations.
Reading between the lines
- The same cycle-removal technique may extend to other combinatorial dichotomies once non-overlapping algebraic-closure conditions can be arranged for the relevant configurations.
- A positive answer to the open question whether every SOP2 theory admits a non-overlapping SOP2 instance (rather than merely a non-overlapping TP instance) would replace the long combinatorial argument by a shorter one that works directly with the standard SOP2-structure.
- The general non-distinctness hypothesis is a purely graph-theoretic statement that can be attacked independently of model theory; a counter-example graph would simultaneously show that the real hierarchy is new and leave the NTP2 collapse open.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper applies the real-valued NSOP_r hierarchy (r>2) to classical questions about the integer-valued NSOP_n hierarchy. Theorem 3.5 shows NSOP_2 ⊆ NSOP_r for 2<r<3 (using NSOP_1=NSOP_2 and Kim-independence), so the intermediate properties are well-defined and newness of all non-integer NSOP_r would separate NSOP_2 from NSOP_3. Theorem 4.4 gives an approximate alternative: if the real and integer hierarchies fail to be distinct on sufficiently general combinatorial grounds (Hypothesis 4.1: every quantifier-free NSOP_n graph with an infinite chain embeds every ≤(n-1)-cycle-free digraph), then NSOP_n ∩ NTP_2 = NSOP_{n+1} ∩ NTP_2 for n≥3. The longest section proves a sharp dichotomy (Theorem 5.5): for a hereditary class H defined by finitely many forbidden weakly embedded substructures, if every theory of age H has SOP_2 then every such theory has SOP_3; Observation 5.6 shows the same fails when SOP_2 is replaced by TP. The proof reduces via Bodirsky–Bodor–Marimon preservation and Cherlin–Shelah–Shi generics to a combinatorial cycle-removal argument (helix maps, split cycles, potentially pinched alternating cycles) inside a carefully constructed age H_std of sound L_std-structures.
Significance. The work supplies three concrete advances on longstanding open problems (NSOP_2 vs NSOP_3; strictness of NSOP_n inside NTP_2) by importing techniques developed for the real-valued hierarchy. The dichotomy of Theorem 5.5 is the first unconditional combinatorial restriction of this strength on ages defined by finitely many forbidden weak embeddings; its sharpness is witnessed by known non-simple NSOP_1 examples. The approximate alternative of Theorem 4.4 links two previously unrelated questions and isolates a purely combinatorial hypothesis whose verification would settle the NTP_2 case. The intermediate fine structure between NSOP_2 and NSOP_3 (Theorem 3.5) is a clean, self-contained contribution that makes the real-valued hierarchy available for further applications. The arguments are fully detailed and rest on cited black-box results rather than circular reasoning.
minor comments (4)
- The multi-step cycle-removal argument in Section 5 (especially the bookkeeping of algebraic closures in Lemmas 5.18 and 5.23 and the three-step removal of split/potentially-pinched alternating cycles) is extremely long; a short roadmap paragraph at the beginning of the combinatorial phase would help the reader track the reductions.
- Several sidebars (NTP_2 graph theory, approximate implications) are interesting but interrupt the main narrative; consider moving them to an appendix or flagging them more clearly as optional.
- Notation for o-maximality / n-o-maximality is introduced late (Definition 4.7) after the concept has already been used informally; a forward pointer would improve readability.
- In the proof of Theorem 3.5 the appeal to symmetry of Kim-independence is noted as optional (footnote); making the coheir-Morley-sequence construction fully self-contained would remove any residual dependence on that fact.
Circularity Check
Minor load-bearing self-citation of prior NSOP1=NSOP2 and cycle-removal techniques; new dichotomies do not reduce by construction to those inputs.
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self citation load bearing
[Theorem 3.5 / proof of Theorem 1.5 (Section 3)]
"Our proof of Theorem 1.5 appears to require the theorem, proven in [25], that NSOP2 = NSOP1. After applying this theorem, we must use analogues of the original stability-theoretic tools... Namely, we apply the theory of Kim-independence in NSOP1 theories..."
The inclusion NSOP2 ⊆ NSOPr for 2<r<3 is load-bearing for the claim that newness of all non-integer NSOPr would separate NSOP2 from NSOP3. The proof invokes the author's prior equality NSOP1=NSOP2 as an indispensable black box before applying Kim-independence. This is ordinary sequential self-citation of a published theorem, not a reduction of the present claim to an unverified internal uniqueness statement; it does not make the later combinatorial dichotomies circular.
full rationale
This is a pure classification-theory paper with no fitted parameters, no empirical predictions, and no self-definitional loops of the form 'X is defined via Y then used to derive Y'. The three main results (NSOP2 ⊆ NSOPr for r>2; the approximate alternative under general non-distinctness; the SOP2–SOP3 dichotomy for ages defined by finitely many forbidden weak embeddings) are proved by new combinatorial constructions (o-maximality via Kim-independence; embedding of the TP2 configuration under Hypothesis 4.1; non-overlapping algebraic closures + adapted helix-map cycle removal for split/potentially-pinched alternating cycles). Prior work by the same author ([25] NSOP1=NSOP2; [26] integrality of o(H) and abstract cycle-removal for helix maps) and external results (Kaplan–Ramsey, Bodirsky–Bodor–Marimon preservation, Cherlin–Shelah–Shi model companions) are cited as black boxes. The only mild self-citation load is that Theorem 3.5 explicitly requires Fact 3.6 from [25]; that fact is an independent published theorem, not an unverified uniqueness claim internal to the present derivation. The cycle-removal arguments of Section 5 are substantially more intricate adaptations, not renamings. No step equates a claimed output to an input by definition. Score 1 reflects the single load-bearing self-citation that is not circular in the sense of the rubric.
Assumptions & free parameters
assumptions (5)
- domain assumption NSOP1 = NSOP2 (author's prior theorem)
- domain assumption Kaplan-Ramsey theory of Kim-independence in NSOP1 theories (symmetry and independence theorem)
- domain assumption Bodirsky-Bodor-Marimon preservation of simplicity and NSOP_n under model companions
- domain assumption Existence of model companions for theories of structures omitting finitely many weak embeddings (Cherlin-Shelah-Shi)
- standard math Standard first-order logic, compactness, indiscernibles, coheir Morley sequences
invented entities (3)
-
o-maximality / n-o-maximality / asymptotic o-maximality
-
presidium, split cycle, alternating cycle, potentially pinched alternating cycle
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Hypothesis 4.1 (non-distinctness on sufficiently general grounds)
Cite this review
Pith. "Pith review of Some applications of the real strict order property hierarchy." pith.science (2026). https://pith.science/paper/H6OKM3E4
@misc{pith2026260628740,
author = {Pith},
title = {Pith review of: Some applications of the real strict order property hierarchy},
year = {2026},
howpublished = {\url{https://pith.science/paper/H6OKM3E4}},
note = {Machine review of arXiv:2606.28740}
}
abstract
We give applications of the properties $\mathrm{NSOP}_{r}$ for non-integer values of $r$ to problems on the original hierarchy $\mathrm{NSOP}_{n}$ for integer values of $n$. We first show that the properties $\mathrm{NSOP}_{r}$, previously defined for real values $r \geq 3$, are even well-defined for real values $r \geq 2$, showing that $\mathrm{NSOP}_{2} \subseteq \mathrm{NSOP}_{r}$ for our original definition of $\mathrm{NSOP}_{r}$ even when $2 < r < 3$. As a consequence, newness of all of the well-defined properties $\mathrm{NSOP}_{r}$ for non-integer $r$ would negatively resolve the problem of whether $\mathrm{NSOP}_{2}$ is equal to $\mathrm{NSOP}_{3}$. We then prove an approximate alternative between two possibilities: (1) that in extending Shelah's original $\mathrm{NSOP}_{n}$ hierarchy for integers $n \geq 3$ to the $\mathrm{NSOP}_{r}$ hierarchy for reals $r > 2$, we really did introduce new classification-theoretic properties, and (2) that $\mathrm{NSOP}_{n+1} \cap \mathrm{NTP}_{2} = \mathrm{NSOP}_{n} \cap \mathrm{NTP}_{2}$ for integers $n \geq 3$, which would resolve a central open problem in classification theory. More precisely, we give a rigorous sense in which (1) can fail on particularly general grounds, and then show that if (1) fails for these general reasons, (2) must be true. Finally, we apply cycle-removal techniques from the theory of the properties $\mathrm{NSOP}_{r}$ for real-values of $r$ to make progress on the question of whether $\mathrm{NSOP}_{2}$ is equal to $\mathrm{NSOP}_{3}$. We (a) show that if $\mathcal{H}$ is a hereditary class of structures defined by finitely many forbidden weakly embedded substructures, if every theory whose models have age $\mathcal{H}$ has $\mathrm{SOP}_{2}$, then every theory whose models have age $\mathcal{H}$ has $\mathrm{SOP}_{3}$, and (b) observe that we cannot replace $\mathrm{SOP}_{2}$ with $\mathrm{TP}$ here.
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Reviewed July 12, 2026 · model on record in the stance chip above.
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