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## QED symbol for end of proof

The amssymb $\LaTeX$ package defines a default Halmos symbol that automatically appears when you use the proof environment. That default Halmos symbol is $\Box$. But if you want that to be filled with black, you could redefine the Halmos symbol. Put the following in your preamble:

\renewcommand{\qed}{\hfill \mbox{\raggedright \rule{0.1in}{0.1in}}}

Better still, you could customize what symbol you want to use as your QED symbol. Just replace the macro \rule{0.1in}{0.1in} with the the symbol of your choice.