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arxiv: 2408.01345 · v1 · pith:E6GAMX47new · submitted 2024-08-02 · 🧮 math.RA · math-ph· math.CO· math.MP· math.QA

On the sub-adjacent Hopf algebra of the universal enveloping algebra of a post-Lie algebra

Pith reviewed 2026-05-25 08:29 UTC · model grok-4.3

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keywords post-Lie algebrapost-Hopf algebrasub-adjacent Hopf algebraantipode formulaOudom-Guin isomorphismGrossman-Larson Hopf algebraordered treestree grafting
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The pith

By twisting the post-Hopf product a combinatorial antipode formula is obtained for the sub-adjacent Hopf algebra of the universal enveloping algebra of a post-Lie algebra.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows how twisting the post-Hopf product produces an explicit combinatorial antipode for the sub-adjacent Hopf algebra that any cocommutative post-Hopf algebra carries. It further supplies a closed-form inverse to the Oudom-Guin isomorphism in the post-Lie setting. As a direct consequence the same method yields a cancellation-free antipode for the Grossman-Larson Hopf algebra on ordered trees expressed by tree grafting. A reader would care because these formulas replace abstract existence statements with concrete combinatorial expressions that can be used for explicit calculations.

Core claim

By twisting the post-Hopf product, we provide a combinatorial antipode formula for the sub-adjacent Hopf algebra of the universal enveloping algebra of a post-Lie algebra. Relating to such a sub-adjacent Hopf algebra, we also obtain a closed inverse formula for the Oudom-Guin isomorphism in the context of post-Lie algebras. Especially as a byproduct, we derive a cancellation-free antipode formula for the Grossman-Larson Hopf algebra of ordered trees through a concrete tree-grafting expression.

What carries the argument

The twisting operation applied to the post-Hopf product, which induces the sub-adjacent Hopf algebra equipped with the generalized Grossman-Larson product.

If this is right

  • The sub-adjacent Hopf algebra of any post-Lie enveloping algebra admits an explicit combinatorial antipode.
  • A closed inverse formula for the Oudom-Guin isomorphism becomes available in the post-Lie case.
  • The Grossman-Larson Hopf algebra on ordered trees receives a cancellation-free antipode expressed by grafting operations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same twisting technique may produce explicit antipodes in other combinatorial Hopf algebras built from similar products.
  • Explicit tree-grafting expressions could be used to compute characters or invariants in algebraic combinatorics on trees.
  • The closed inverse for the Oudom-Guin map might simplify change-of-basis calculations between different enveloping constructions.

Load-bearing premise

The twisting operation preserves the cocommutative post-Hopf algebra axioms and produces a well-defined sub-adjacent Hopf algebra structure.

What would settle it

Direct substitution of the proposed antipode formula into the defining relation of the sub-adjacent Hopf algebra for a concrete low-dimensional post-Lie algebra and checking whether the product is inverted.

read the original abstract

Recently the notion of post-Hopf algebra was introduced, with the universal enveloping algebra of a post-Lie algebra as the fundamental example. A novel property is that any cocommutative post-Hopf algebra gives rise to a sub-adjacent Hopf algebra with a generalized Grossman-Larson product. By twisting the post-Hopf product, we provide a combinatorial antipode formula for the sub-adjacent Hopf algebra of the universal enveloping algebra of a post-Lie algebra. Relating to such a sub-adjacent Hopf algebra, we also obtain a closed inverse formula for the Oudom-Guin isomorphism in the context of post-Lie algebras. Especially as a byproduct, we derive a cancellation-free antipode formula for the Grossman-Larson Hopf algebra of ordered trees through a concrete tree-grafting expression.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 3 minor

Summary. The paper studies post-Hopf algebras, noting that any cocommutative post-Hopf algebra induces a sub-adjacent Hopf algebra equipped with a generalized Grossman-Larson product. By twisting the post-Hopf product on the universal enveloping algebra of a post-Lie algebra, it derives a combinatorial antipode formula for the resulting sub-adjacent Hopf algebra, a closed-form inverse to the Oudom-Guin isomorphism, and (as a byproduct) a cancellation-free antipode for the Grossman-Larson Hopf algebra on ordered trees expressed via explicit tree-grafting operations.

Significance. If the twisting construction preserves the required axioms and the explicit formulas are verified, the work supplies concrete combinatorial tools for antipodes and isomorphisms in post-Lie and tree-based Hopf algebras, strengthening the link between post-Lie structures and algebraic combinatorics.

minor comments (3)
  1. The abstract invokes the twisting operation and the preservation of cocommutative post-Hopf axioms without stating the explicit form of the twist or the verification that the new product remains associative and coassociative; this definition and check should appear in §2 or §3 before the main theorems.
  2. The tree-grafting expression for the Grossman-Larson antipode is described as 'concrete' but is not displayed in the abstract or early sections; an explicit formula (perhaps as a displayed equation) should be given in the introduction or the relevant theorem statement.
  3. Notation for the post-Hopf product, the twist, and the sub-adjacent product should be introduced with a short table or list of definitions to avoid ambiguity when multiple products appear in the same proof.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive summary and significance assessment of our work deriving combinatorial antipode formulas via product twisting in post-Hopf algebras, along with the closed inverse for the Oudom-Guin isomorphism and the cancellation-free antipode for ordered trees. The recommendation for minor revision is noted.

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper constructs a sub-adjacent Hopf algebra by twisting the post-Hopf product on the universal enveloping algebra of a post-Lie algebra, then derives combinatorial antipode formulas and an inverse to the Oudom-Guin isomorphism as direct algebraic consequences. No steps reduce by definition to their own inputs, no parameters are fitted and relabeled as predictions, and no load-bearing claims rest on self-citations whose content is itself unverified or circular. The derivation chain consists of explicit operations (twisting, grafting) on known structures whose verification is internal to the algebraic identities presented, rendering the result self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Paper operates entirely within standard definitions of post-Lie algebras, post-Hopf algebras, and Hopf algebra antipodes drawn from prior literature; no free parameters, ad-hoc axioms, or new entities are indicated in the abstract.

axioms (1)
  • domain assumption Standard axioms of post-Lie algebras and cocommutative post-Hopf algebras hold and allow twisting to produce a sub-adjacent Hopf algebra.
    Invoked when stating that any cocommutative post-Hopf algebra gives rise to a sub-adjacent Hopf algebra with generalized Grossman-Larson product.

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