Definition:Halmos Symbol/Historical Note
Historical Note on Halmos Symbol
However, he is generally credited with introducing it into mathematics, having seen it used in general magazine literature to indicate the end of an article.
In his own words:
- The symbol is definitely not my invention — it appeared in popular magazines (not mathematical ones) before I adopted it, but, once again, I seem to have introduced it into mathematics. It is the symbol that sometimes looks like $\Box$, and is used to indicate an end, usually the end of a proof. It is most frequently called the 'tombstone', but at least one generous author referred to it as the 'halmos'.