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Q.E.D. is an abbreviation of the Latin phrase "quod erat demonstrandum" (literally, "which was to be demonstrated"). The phrase is written in its abbreviated form at the end of a mathematical proof or philosophical argument, to signify that the last statement deduced was the one to be demonstrated, so the proof is complete.

The phrase is a translation into Latin of the original Greek ὅπερ ἔδει δει̂ξαι. (hoper edei deixai) which was used by many early mathematicians including Euclid^[1] and Archimedes. These mathematicians, in particular Euclid, are credited with founding axiomatic mathematics with its emphasis on establishing truths by logical deduction (rather than experimentation or assertion); their use of this phrase symbolizes this emphasis, as well as marking this important step in the development of mathematical philosophy.

Perhaps the most famous use of Q.E.D. in a philosophical argument is found in the Ethics of Baruch Spinoza, published posthumously in 1677. Written in Latin, it is considered his magnum opus.

The style and system of the book is, as Spinoza says, "demonstrated in geometrical order", with axioms and definitions followed by propositions. For Spinoza, this is a considerable improvement over René Descartes's writing style in the Meditations, which follows the form of a diary.^[2]

Currently, it has become so symbolic of irrefutable logic that "Q.E.D." is occasionally used in non-mathematical contexts as well to intensify assertions; in this context it has little connection with rigorous deduction, however, and is more tongue-in-cheek.

There is another Latin phrase with a slightly different meaning, and less common in usage. Quod erat faciendum is translated as "which was to be done." This is usually shortened to Q.E.F.. As with Q.E.D., Q.E.F. is a translation of the Greek geometers' closing ὅπερ ἔδει ποιῆσαι (hoper edei poiēsai). Euclid used this phrase to close propositions which were not precisely "proofs", but rather exemplar constructions. The distinction between Q.E.D. and Q.E.F. is roughly equivalent to the distinction between a proof and an illustration of the proof.

As mathematics has come to be written in languages other than Latin, Q.E.D. has acquired many translations; in French and German (two languages a mathematician might see often) it is respectively C.Q.F.D, for "ce qu'il fallait démontrer", and W.Z.B.W, for "was zu beweisen war". There does not appear to be a common English equivalent, though the end of a proof may be announced with a simple statement such as "this completes the proof" or a similar locution.

With computers frequently being used to "write" proofs (see LaTeX), there are several symbolic alternatives in use. The most popular symbol is ■ (solid black square), also called tombstone or Halmos symbol (after Paul Halmos, who pioneered its use). The tombstone is sometimes open: □ (hollow black square). Unicode explicitly provides the "End of Proof" character U+220E (∎), but also offers ▮ (U+25AE, black vertical rectangle) and ‣ (U+2023, triangular bullet) as alternatives. Some authors have adopted variants of this notation with other symbols, such as two forward slashes (//), or simply some vertical white space.

1. ^ Elements 2.5 by Euclid (ed. J. L. Heiberg), retrieved 16 July 2005
2. ^ The Chief Works of Benedict De Spinoza, translated by R. H. M. Elwes, 1951 (available online - links found on the Wikipedia page Ethics (book)).

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