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Under reflexivity, the two Euclidean forms of the zeroth law are equivalent

Proved
ZerothLawThermo.zerothLaw_iff_zerothLawPlanck_of_refl

by Lucas · Oct 9, 2026 · Mathlib 0df444a (Lean v4.33.1)

equivalence-relationthermodynamicszeroth-law

Let SSS be a type of thermodynamic systems and TE\mathrm{TE}TE a binary relation on SSS such that TE(A,A)\mathrm{TE}(A,A)TE(A,A) for all A∈SA\in SA∈S. Then the following are equivalent:

  1. the zeroth law (right-Euclidean form): TE(A,C)∧TE(B,C)⇒TE(A,B)\mathrm{TE}(A,C)\wedge\mathrm{TE}(B,C)\Rightarrow\mathrm{TE}(A,B)TE(A,C)∧TE(B,C)⇒TE(A,B) for all A,B,CA,B,CA,B,C;
  2. Planck's form (left-Euclidean form): TE(C,A)∧TE(C,B)⇒TE(A,B)\mathrm{TE}(C,A)\wedge\mathrm{TE}(C,B)\Rightarrow\mathrm{TE}(A,B)TE(C,A)∧TE(C,B)⇒TE(A,B) for all A,B,CA,B,CA,B,C.

This justifies the source's passage from Planck's statement to the now-standard statement of the law, once reflexivity is assumed.

Preamble
import Definitions.Def_ZerothLawThermo_Defs
import Mathlib

open ZerothLawThermo
Formal statement
theorem ZerothLawThermo.zerothLaw_iff_zerothLawPlanck_of_refl {S : Type*} (TE : S → S → Prop)
    (hrefl : ∀ A : S, TE A A) :
    ZerothLaw TE ↔ ZerothLawPlanck TE := by sorry
Source
Wikipedia, "Zeroth law of thermodynamics", revision oldid=1328026962, https://en.wikipedia.org/w/index.php?title=Zeroth_law_of_thermodynamics&oldid=1328026962; Section "Equivalence relation": Planck's left-Euclidean statement, then "Thus, again implicitly assuming reflexivity, the zeroth law is therefore often expressed as a right-Euclidean statement: If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other."
Read-back

What the Lean code literally says, in plain math · Aristotle (Harmonic) — same agent as the drafter; non-blind

Disclosure — this read-back is NOT blind. It was written by the same agent (Aristotle, by Harmonic) that drafted these Lean statements, with full knowledge of the source text and of the intended meaning. It is therefore not independent testimony and must not be mistaken for an independent audit; reviewers should check the Lean code against the source themselves.

For an arbitrary type SSS (any universe, possibly empty) and an arbitrary binary relation TE\mathrm{TE}TE on SSS with TE(A,A)\mathrm{TE}(A,A)TE(A,A) for every A∈SA\in SA∈S, the statement asserts the logical equivalence

ZerothLaw(TE)  ⟺  ZerothLawPlanck(TE),\texttt{ZerothLaw}(\mathrm{TE})\iff\texttt{ZerothLawPlanck}(\mathrm{TE}),ZerothLaw(TE)⟺ZerothLawPlanck(TE),

where the left side means: for all A,B,CA,B,CA,B,C, TE(A,C)∧TE(B,C)⇒TE(A,B)\mathrm{TE}(A,C)\wedge\mathrm{TE}(B,C)\Rightarrow\mathrm{TE}(A,B)TE(A,C)∧TE(B,C)⇒TE(A,B); and the right side means: for all A,B,CA,B,CA,B,C, TE(C,A)∧TE(C,B)⇒TE(A,B)\mathrm{TE}(C,A)\wedge\mathrm{TE}(C,B)\Rightarrow\mathrm{TE}(A,B)TE(C,A)∧TE(C,B)⇒TE(A,B). Reflexivity is the only hypothesis; both directions of the equivalence are asserted.

Human review
  • Endorsed by Shuze Chen · Oct 10, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Lucas · Oct 10, 2026

    Confirmed by the mission captain (proposal self-audit).

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