Read Tombstone (typography) (Wikipedia)
In mathematics, the tombstone, halmos, end-of-proof, or Q.E.D. symbol "∎" (or "□") is a symbol used to denote the end of a proof, in place of the traditional abbreviation "Q.E.D." for the Latin phrase "quod erat demonstrandum", meaning "which was to be demonstrated". In magazines, it is one of the various symbols used to indicate the end of an article. In Unicode, it is represented as character U+220E ∎ END OF PROOF (HTML ∎). Its graphic form varies, as it may be a hollow or filled rectangle or square.

Paul Halmos on Prerequisites

Definitely the quote of the day:

Paul Halmos (1916 – 2006, Hungarian-born American mathematician
in Measure Theory (1950)


This is essentially the mathematician’s equivalent of the adage “Fake it ’til you make it.”