Joint Projective Invariants on First Jet Spaces of Point Configurations via Moving Frames

TL;DR

This work studies the action of $PGL(3,\mathbb{R})$ on the $n$-fold first-order jet space of planar point configurations and, using the moving frames method, produces a complete generating set for the absolute first-order joint projective differential invariants $\mathcal{I}_{n,0}$ for all $n\ge3$. It also characterizes the relative invariants $\mathcal{I}_n$ as a simple extension of $\mathcal{I}_{n,0}$, with the primitive element given by the invariantization of the Jacobian; an explicit formula for this element is derived and rational for all $n$. Furthermore, the paper constructs a multiplicative cochain complex for the $G$-action and provides a contracting homotopy, yielding a constructive proof of the vanishing of higher cohomology. The results unify the invariant theory across all $n$ and supply practical, numerically stable formulas for invariants and invariantized multipliers, with potential applications in projective-invariant image descriptors and pattern recognition.

Abstract

We consider the action of the projective group $PGL(3,\mathbb{R})$ on the $n$-fold first-order jet space of point configurations on the plane. Using the method of moving frames, we construct an explicit complete generating set for the field of absolute first-order joint projective differential invariants $\mathcal{I}_{n,0}$ for any $n \ge 3$. This approach provides a unified construction for all $n$, immediately ensuring functional independence of the fundamental invariants and yielding formulas suitable for both symbolic and numerical implementation. Next, we study the field of relative first-order invariants $\mathcal{I}_n$ with Jacobian multiplier. It is shown that the invariantization of the Jacobian under the projective action yields a primitive element of the field extension $\mathcal{I}_n / \mathcal{I}_{n,0}$. Finally, we introduce a multiplicative cochain complex $C^\bullet$ associated with the action of $PGL(3,\mathbb{R})$ on the jet space, and show that the invariantization operator induced by the moving frame generates an explicit contracting homotopy. This provides a constructive proof of the vanishing of higher cohomology and an interpretation of the "defect" of invariantization as an exact cocycle in $C^\bullet$.

Theorems & Definitions (12)

• Theorem 1
• proof
• Theorem 2
• Theorem 3
• Theorem 4
• proof
• Theorem 5
• proof
• Theorem 6
• proof