The Legendre transform is a mathematical involution that maps convex functions to convex functions while preserving complete information, which is why it is foundational for switching between equivalent thermodynamic potentials (such as energy and entropy) without loss of data.
The claim states a fundamental mathematical property of the Legendre transform—that it is a dual transformation preserving all information (invertible under convexity conditions), commonly taught and used in physics and mathematics as an isomorphism between conjugate variables. Because this is a definitional mathematical property, it falls under common knowledge and does not require empirical citation.