Full Idea
The 'Tractatus' does not allow the introduction of a sign for identity.
Gist of Idea
The sign of identity is not allowed in 'Tractatus'
Source
report of Ludwig Wittgenstein (Tractatus Logico-Philosophicus [1921]) by David Bostock - Philosophy of Mathematics 9.B.4
Book Reference
Bostock,David: 'Philosophy of Mathematics: An Introduction' [Wiley-Blackwell 2009], p.286