Q.E.D.

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Q.E.D. is an abbreviation of the Latin phrase quod erat demonstrandum, which literally means "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; the abbreviation thus signals the completion of the proof.

Etymology and early use

The phrase is a translation into Latin from Greek ὅπερ ἔδει δεῖξαι (hoper edei deixai; abbreviated as ΟΕΔ), a phrase 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.

Modern philosophy

Philippe van Lansberge's 1604 Triangulorum Geometriæ used quod erat demonstrandum to conclude some proofs; others ended with phrases such as sigillatim deinceps demonstrabitur, magnitudo demonstranda est, and other variants.^[2]

In the European Renaissance, scholars often wrote in Latin, and phrases such as Q.E.D. were often used to conclude proofs.

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 by many to be Spinoza's 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.^[3]

Q.E.F.

There is another Latin phrase with a slightly different meaning, and less common in usage. Quod erat faciendum is translated as "which was to have been done". This is usually shortened to Q.E.F. The expression quod erat faciendum is a translation of the Greek geometers' closing ὅπερ ἔδει ποιῆσαι (hoper edei poiēsai). Euclid used this phrase to close propositions which were not proofs of theorems, but constructions. For example, Euclid's first proposition shows how to construct an equilateral triangle given one side.

Equivalents in other languages

Q.E.D. has acquired many translations in various foreign languages. In French, German, Italian and Russian (with English, the main languages of modern Western mathematics) it is respectively C.Q.F.D., for ce qu'il fallait démontrer (or sometimes ce qui finit la démonstration), W.Z.B.W. for was zu beweisen war, C.V.D. for come volevasi dimostrare, and ч.т.д., for что и требовалось доказать. In Spanish it is Q.E.D. for Quedo esto demostrado, and in Portuguese it is C.Q.D. for Conforme queríamos demostrar. There does not appear to be a common formal 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. Most modern math textbooks in English end proofs with a symbol, often square. (See below.) In modern Greek texts sometimes the initials ο.ε.δ. (for ὅπερ ἔδει δεῖξαι) are used at the end of a mathematical proof. In Hebrew מ.ש.ל מה שצריך למצוא.

Electronic forms

When typesetting was done by a compositor with letterpress printing, complex typography such as mathematics and foreign languages were called "penalty copy" (the author paid a "penalty" to have them typeset, as it was harder than plain text). With the advent of systems such as LaTeX, mathematicians found their options more open, so there are several symbolic alternatives in use, either in the input, the output, or both. When creating TeX, Knuth provided the symbol ■ (solid black square), also called by mathematicians 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.

In popular culture

This "In popular culture" section may contain too many minor or trivial references. Please reorganize this content to explain the subject's impact on popular culture rather than simply listing appearances, and remove trivia references. (November 2009)

The 1982 US television series Q.E.D. starred Sam Waterston as Professor Quentin Everett Deverill, an American detective in Edwardian England who uses a style of logical deduction similar to that of Sherlock Holmes. Hence, the show's title derives both from the protagonist's initials (by which he is primarily known), as well as the logical proofs he presents.^{[citation needed]}

Douglas Adams' franchise, The Hitchhiker's Guide to the Galaxy, famously uses Q.E.D. to conclude its Babel fish proof, which determines that God no longer exists because the Babel fish is too improbable to have evolved by pure chance; therefore the Babel fish was a proof for God, and as Faith involves no proof, there was no God, QED (using humorous fallacy to mock teleology and intelligent design principles).^[4]

In the 1987 film No Way Out starring Gene Hackman, Kevin Costner, Sean Young and with Will Patton, Fred Thompson and Brad Pitt, Patton's character shouts the phrase during the film's climax, figuring he has solved the spy mystery in the Defense Secretary's office moments before Hackman's and Costner's characters scuffle and Patton subsequently commits suicide.

References

External links

Look up quod erat demonstrandum in Wiktionary, the free dictionary.

• Earliest Known Uses of Some of the Words of Mathematics (Q)
• Robin Hartshorne note on QED