Àwọn tábìlì yìí ṣe àkójọ ọ̀pọ̀lọpọ̀ àwọn ìdánimọ̀ tí a ma ń sábà lọ́ ní ìmọ̀ ìṣirò, tí a tò wọ́n gẹ́gẹ́ bí àkókò tí ṣàfihàn rẹ̀. Weil, André (1992), The Apprenticeship of a Mathematician, Springer, p. 114, ISBN 9783764326500 . Halmos, Paul (1950). Measure Theory. New York: Van Nostrand. pp. vi. "The symbol ∎ is used throughout the entire book in place of such phrases as "Q.E.D." or "This completes the proof of the theorem" to signal the end of a proof."  Kenneth E. Iverson (1962), A Programming Language, Wiley, retrieved 20 April 2016