Infinitesimal Legendre symmetry in the Geometrothermodynamics programme

TL;DR

This paper investigates metrics in geometrothermodynamics that are invariant under infinitesimal Legendre transformations, analyzing their properties and implications for thermodynamic systems like the ideal gas.

Contribution

It introduces and solves the Legendre-Killing equation for a class of contact metrics, exploring their invariance and impact on thermodynamic curvature interpretations.

Findings

Unique solutions for Legendre-invariant metrics under certain conditions

Scalar curvature cannot be used as a measure of interaction in this class

Constraints restrict the metric functions compatible with Legendre invariance

Abstract

The work within the Geometrothermodynamics programme rests upon the metric structure for the thermodynamic phase-space. Such structure exhibits discrete Legendre symmetry. In this work, we study the class of metrics which are invariant along the infinitesimal generators of Legendre transformations. We solve the Legendre-Killing equation for a $K$-contact general metric. We consider the case with two thermodynamic degrees of freedom, i.e. when the dimension of the thermodynamic phase-space is five. For the generic form of contact metrics, the solution of the Legendre-Killing system is unique, with the sole restriction that the only independent metric function -- $\Omega$ -- should be dragged along the orbits of the Legendre generator. We revisit the ideal gas in the light of this class of metrics. Imposing the vanishing of the scalar curvature for this system results in a further differential equation for the metric function $\Omega$ which is not compatible with the Legendre invariance constraint. This result does not allow us to use the regular interpretation of the curvature scalar as a measure of thermodynamic interaction for this particular class.