Full Idea
Unlike ∨, →, ↔, and ∀, the sign = is not eliminable from a logic.
Clarification
=
Gist of Idea
Unlike most other signs, = cannot be eliminated
Source
Engelbretsen,G/Sayward,C (Philosophical Logic: Intro to Advanced Topics [2011], Ch.3)
Book Reference
Engelbretsen,G/Sayward,C: 'Philosophical Logic: Intro to Advanced Topics' [Continuum 2011], p.70