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arxiv: 2604.20345 · v1 · submitted 2026-04-22 · 💻 cs.LO

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A Rocq Formalization of Simplicial Lagrange Finite Elements

CERMICS UMR 9032), Fran\c{c}ois Cl\'ement (SERENA, Houda Mouhcine (TOCCATA, LIPN, LIPN), Micaela Mayero (TOCCATA, SERENA, Sylvie Boldo (TOCCATA), Vincent Martin (LMAC)

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Pith reviewed 2026-05-09 23:27 UTC · model grok-4.3

classification 💻 cs.LO
keywords Rocqformal verificationfinite element methodLagrange elementsunisolvencesimplicialmultivariate polynomialsaffine spaces
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The pith

The paper defines a finite element as a Rocq record and instantiates it with simplicial Lagrange elements of any dimension and degree, proving unisolvence.

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

The paper designs a finite element as a record in the Rocq proof assistant containing both the geometric and polynomial data and the proofs that these data satisfy the required properties. The key property is unisolvence, which ensures that the linear forms uniquely determine the polynomials. This formalization is then applied to the simplicial Lagrange finite elements with evenly distributed nodes, covering any dimension and any polynomial degree. A sympathetic reader would care because this step advances the long-term goal of proving the correctness of programs that use the finite element method to solve partial differential equations.

Core claim

A finite element is defined in Rocq as a record that includes the cell geometry, the polynomial approximation space, and a set of linear forms, together with proofs that the space has the correct dimension and that the forms are unisolvent. The record is instantiated with simplicial Lagrange finite elements for any dimension and degree, with the unisolvence proof included.

What carries the argument

The Rocq record for a finite element, which packages the cell vertices, the polynomial space, the degrees of freedom, and the validity proofs including unisolvence.

If this is right

  • The formal definition ensures that the Lagrange elements satisfy all mathematical requirements for any dimension and degree.
  • Supporting results on polynomials in finite and infinite-dimensional spaces are available as reusable Rocq components.
  • The unisolvence property is established even for the general case, which is noted as difficult.
  • This provides a basis for proving properties of finite element methods in a proof assistant.

Where Pith is reading between the lines

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

  • This formalization advances the goal of proving correctness of scientific computing programs using the finite element method.
  • The lemmas developed can support formal proofs of other aspects of numerical analysis.
  • Generalization to non-simplicial elements or different node distributions would build directly on the record structure.
  • Once local cells are formalized, global mesh assembly and solution procedures become amenable to formal verification.

Load-bearing premise

The Rocq record definitions and supporting lemmas about polynomials and affine spaces accurately capture the intended mathematical concepts of finite elements.

What would settle it

For a specific simplex in dimension 2 with polynomial degree 1, the formalized linear forms would fail to be unisolvent if the proof does not establish that any polynomial vanishing on all nodes is the zero polynomial.

Figures

Figures reproduced from arXiv: 2604.20345 by CERMICS UMR 9032), Fran\c{c}ois Cl\'ement (SERENA, Houda Mouhcine (TOCCATA, LIPN, LIPN), Micaela Mayero (TOCCATA, SERENA, Sylvie Boldo (TOCCATA), Vincent Martin (LMAC).

Figure 1
Figure 1. Figure 1: 3D mesh of a cerebral aneurysm (colors only help to distinguish zones). [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Reference Lagrange finite elements LagP d k in dimension d = 2 (top) and d = 3 (bottom), with degrees of approximation k = 1 (left) and k = 2 (right). The geometric elements are d-simplices. The nodes (depicted by blue dots) are only the vertices when k = 1, and when k = 2, nodes are added in the middle of edges. Their coordinates being real numbers, they are denoted with a point suffix, such as in “0.” an… view at source ↗
Figure 3
Figure 3. Figure 3: Hierarchy of algebraic structures used in the present development. [PITH_FULL_IMAGE:figures/full_fig_p019_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Affine geometric transformation T v in the case d = 2. Given v = (v0, v1, v2), T v maps the reference triangle Kˆ to the triangle K of vertices v. Given d+ 1 current vertices v of a current geometric element K ⊂ R d , an affine mapping send￾ing reference vertices vˆ onto v preserves barycenters (see the definition of aff_map in Section 5.1). Thus, the affine geometric transformation T v outputs the point w… view at source ↗
Figure 5
Figure 5. Figure 5: Injection onto a face hyperplane I v 0 in the case d = k = 3. The reference triangle Kˆ d−1 is first mapped onto the 0-th face of Kˆ d , opposite vertex vˆ0, and then to the 0-th face of Kd , that is contained in the face hyperplane Hv 0 supported by (v1, v2, v3). The hyperplanes are depicted in light blue and the faces in a darker blue. After reading Section 9.1 about nodes, note that the correspondence b… view at source ↗
Figure 6
Figure 6. Figure 6: Geometric transformation T τ 0 d (v) in the case d = k = 3. The transposition τ 0 d maps (0, 1, 2, 3) onto (3, 1, 2, 0). The reference simplex Kˆ d is mapped onto the current simplex K. The reference vertices are mapped onto the current vertices with a change of indices: for all i ∈ [0..d], we have T τ 0 d (v) (vˆi) = vτ 0 d (i) . Note that, as τ 0 d (d) = 0, the reference face hyperplane Hˆd (opposite ref… view at source ↗
Figure 7
Figure 7. Figure 7: Affine transformation of a finite element, from vertices to vertices. [PITH_FULL_IMAGE:figures/full_fig_p038_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Table of inputs and hypotheses for the finite element transformation [PITH_FULL_IMAGE:figures/full_fig_p038_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Lagrange nodes (aˆα)α∈Ad k of the reference simplex Kˆd when d ∈ {2, 3} and k = 3. Each node is depicted as a colored ball, and corresponds to a unique element of A d 3. The colors correspond to degrees l ⩽ 3 of polynomials, or equivalently to the sizes of multi-indices (i.e. in C d l for l ⩽ 3). In magenta, the node aˆ0 corresponds to constant polynomials (with degree 0) in P d 0, and to the multi-index 0… view at source ↗
Figure 10
Figure 10. Figure 10: Partition C d k = Uk i=0{(k − i, β)| β ∈ Cd−1 i }, with d = k = 3. Geometrically, {(k−i, β)| β ∈ Cd−1 i } is the intersection of the hyperplane of equation α0 = k−i (depicted in light pink) and of C d k (of equation α0 + α1 + α2 = k). The reference Lagrange node aˆ(α1,α2,α3) belonging to the face Hˆ0 is in blue and corresponds to the element (α1, α2, α3) ∈ Cd k. For instance, the set {(1, 2, 0),(1, 1, 1),… view at source ↗
Figure 11
Figure 11. Figure 11: Grsymlex ordering of A d k when d ∈ {2, 3} and k = 3. Dashed arrows represent the increase in the order. For A 2 3 (d = 2), we have (0, 0) < grsymlex (1, 0) < grsymlex (0, 1) < grsymlex (2, 0) < grsymlex (1, 1) < grsymlex (0, 2) < grsymlex (3, 0) < grsymlex (2, 1) < grsymlex (1, 2) < grsymlex (0, 3). When d ≥ 3, we have for instance for C 3 3 (3, 0, 0) < grsymlex (2, 1, 0) < grsymlex (2, 0, 1) < grsymlex … view at source ↗
Figure 12
Figure 12. Figure 12: Lagrange nodes of the reference simplex for [PITH_FULL_IMAGE:figures/full_fig_p048_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Passage from the reference subnodes of LagP 3 3 (degree ⩽ 3) to the reference nodes of LagP 3 2 (degree ⩽ 2) in the case d = k = 3. Plot of subvertices (vˆi )i∈[0..d] and subnodes (aˆα′ )α′∈A3 2 . The nodes (aˆα)α∈A3 3 are set in the tetrahedron defined by the vertices (vˆ0, vˆ1, vˆ2, vˆ3). The subvertices are (vˆ0, aˆ2δ0 , aˆ2δ1 , aˆ2δ2 ). The subnodes (aˆα′ )α′∈A3 2 are defined with respect to the tetra… view at source ↗
Figure 14
Figure 14. Figure 14: Various ways to equip R d with the affine space structure. Double-edged boxes denote actual instances of canonical structures in the Rocq system for R, that is to say possible terminal points for the canonical structure search. Rectangular boxes denote the intent to equip a space (either R or R d ) with an algebraic structure. Arrows correspond to generic canonical structures in our library, or in the Coq… view at source ↗
Figure 15
Figure 15. Figure 15: Dependency graph, including the literature (in green), our contributions presented in this [PITH_FULL_IMAGE:figures/full_fig_p059_15.png] view at source ↗
read the original abstract

Formalization of mathematics is a major topic, that includes in particular numerical analysis, towards proofs of scientific computing programs. The present study is about the finite element method, a popular method to numerically solve partial differential equations. In the long-term goal of proving its correctness, we focus here on the formal definition of what is a finite element. Mathematically, a finite element describes what happens in a cell of a mesh. It notably includes the geometry of the cell, the polynomial approximation space, and a finite set of linear forms that computationally characterizes the polynomials. Formally, we design a finite element as a record in the Rocq proof assistant with both values (such as the vertices of the cell) and proofs of validity (such as the dimension of the approximation space). The decisive validity proof is unisolvence, that makes the previous characterization unique. We then instantiate this record with the most popular and useful, the simplicial Lagrange finite elements for evenly distributed nodes, for any dimension and any polynomial degree, including the difficult unisolvence proof. These proofs require many results (definitions, lemmas, canonical structures) about finite families, affine spaces, multivariate polynomials, in the context of finite or infinite-dimensional spaces.

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

1 major / 2 minor

Summary. The paper presents a formalization in the Rocq proof assistant of finite elements as records containing geometric data, polynomial approximation spaces, and degrees of freedom with associated validity proofs, including the crucial unisolvence property. It specializes this to simplicial Lagrange finite elements with evenly distributed nodes, providing the construction and unisolvence proof for arbitrary dimensions and polynomial degrees, along with supporting mathematical libraries for finite families, affine spaces, and multivariate polynomials.

Significance. If the formalization is correct, this work is significant as it provides a machine-checked foundation for the finite element method, which is essential for proving the correctness of numerical solvers for PDEs. The generality to any dimension and degree, with the difficult unisolvence proof, is a notable achievement that can serve as a basis for further formal developments in scientific computing. The explicit machine-checked proofs and reusable library on polynomials and affine spaces are particular strengths.

major comments (1)
  1. The central claim is the instantiation of the finite-element record with a complete unisolvence proof for simplicial Lagrange elements of arbitrary dimension and degree. However, the manuscript describes the record structure and supporting lemmas at a high level without exhibiting the Rocq definition of the record type, the statement of the unisolvence lemma, or key intermediate results on the polynomial space dimension, leaving the load-bearing formalization unverifiable from the text.
minor comments (2)
  1. The abstract and introduction mention 'many results (definitions, lemmas, canonical structures)' on finite families and multivariate polynomials; a dedicated section or appendix listing the most important ones with their Rocq names would improve readability and reusability.
  2. Notation for affine spaces and evenly distributed nodes is introduced without a small concrete example (e.g., the 2D degree-1 or degree-2 case) that would help readers map the general construction to familiar finite-element tables.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their positive evaluation of the significance of our Rocq formalization and for the constructive feedback. We address the single major comment below.

read point-by-point responses
  1. Referee: The central claim is the instantiation of the finite-element record with a complete unisolvence proof for simplicial Lagrange elements of arbitrary dimension and degree. However, the manuscript describes the record structure and supporting lemmas at a high level without exhibiting the Rocq definition of the record type, the statement of the unisolvence lemma, or key intermediate results on the polynomial space dimension, leaving the load-bearing formalization unverifiable from the text.

    Authors: We agree that the manuscript presents the finite-element record and unisolvence proof at a high level. To improve direct verifiability of the central claim, the revised version will include the Rocq definition of the finite element record type, the precise statement of the unisolvence lemma, and key intermediate results establishing the dimension of the polynomial approximation space (including the cardinality of the chosen basis). These will be placed in a dedicated section or appendix, while preserving readability of the main text. The complete development with all proofs is already available in the public Rocq repository linked from the paper. revision: yes

Circularity Check

0 steps flagged

No significant circularity identified.

full rationale

The paper presents a direct Rocq formalization of the standard mathematical definition of a finite element (as a record containing cell geometry, polynomial space, and degrees of freedom together with validity proofs including unisolvence). It instantiates this record for simplicial Lagrange elements of arbitrary dimension and degree using supporting libraries for affine spaces, multivariate polynomials, and finite families. No step reduces a claimed result to its own inputs by definition, no fitted parameters are renamed as predictions, and no load-bearing premise relies on a self-citation chain whose content is unverified outside the present work. The derivation is therefore self-contained as a faithful encoding of externally established mathematics.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper relies on standard mathematical results about polynomials and affine spaces from Rocq libraries without introducing new free parameters or invented entities.

axioms (1)
  • standard math Properties of finite families, affine spaces, and multivariate polynomials
    Used to construct the finite element record and prove unisolvence for general cases.

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Reference graph

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