Almost Equal: the Method of Adequality from Diophantus to Fermat and Beyond Mikhail G. Katz, David M. Schaps, and Steven Shnider Bar Ilan University We analyze some of the main approaches in the literature to the method of ‘adequality’ with which Fermat approached the problems of the calculus, as well as its source in the parisâth§ of Diophantus, and propose a novel read- ing thereof. Adequality is a crucial step in Fermat’s method of ªnding max- ima, minima, tangents, and solving other problems that a modern mathema- tician would solve using inªnitesimal calculus. The method is presented in a series of short articles in Fermat’s collected works (Tannery and Henry 1981, pp. 133–172). We show that at least some of the manifestations of adequality amount to variational techniques exploiting a small, or inªn- itesimal, variation e. Fermat’s treatment of geometric and physical applica- tions suggests that an aspect of approximation is inherent in adequality, as well as an aspect of smallness on the part of e. We question the relevance to understanding Fermat of 19th century dictionary deªnitions of parisâth§ and adaequare, cited by Breger, and take issue with his interpretation of adequality, including his novel reading of Diophantus, and his hypothesis concerning alleged tampering with Fermat’s texts by Carcavy. We argue that Fermat relied on Bachet’s reading of Diophantus. Diophantus coined the term parisâth§ for mathematical purposes and used it to refer to the way in which 1321/711 is approximately equal to 11/6. Bachet performed a se- mantic calque in passing from parisoo to adaequo. We note the similar role of, respectively, adequality and the Transcendental Law of Homogeneity in the work of, respectively, Fermat (1896) and Leibniz (1858) on the problem of maxima and minima. We are grateful to Enrico Giusti, Alexander Jones, and David Sherry for helpful com- ments. The inºuence of Hilton Kramer (1928–2012) is obvious. Perspectives on Science 2013, vol. 21, no. 3 ©2013 by The Massachusetts Institute of Technology doi:10.1162/POSC_a_00101 283 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00101 by guest on 01 October 2021 284 The Method of Adequality 1. The Debate over Adequality Adequality, or â (parisotes) in the original Greek of Diophantus1,is a crucial step in Fermat’s method of ªnding maxima, minima, tangents, and solving other problems that a modern mathematician would solve using inªnitesimal calculus. The method is presented in a series of short articles in Fermat’s collected works (1891, pp. 133–172). The ªrst article, Methodus ad Disquirendam Maximam et Minimam2, opens with a summary of an algorithm for ªnding the maximum or minimum value of an algebraic expression in a variable A. For convenience, we will write such an expres- sion in modern functional notation as f(a).3 1.1. Summary of Fermat’s Algorithm The algorithm can be broken up into six steps in the following way: 1. Introduce an auxiliary symbol e, and form f(a ϩ e); 4 2. Set adequal the two expressions f(a ϩ e) ϭAD f(a); 3. Cancel the common terms on the two sides of the adequality. The remaining terms all contain a factor of e; 4. Divide by e (see also next step); 5. In a parenthetical comment, Fermat adds: “or by the highest com- mon factor of e”; 6. Among the remaining terms, suppress5 all terms which still con- tain a factor of e.6 Solving the resulting equation for a yields the extremum of f. In modern mathematical language, the algorithm entails expanding the difference quotient fa()()+− e fa e 1. adaequalitas or adaequare in Latin. See Section 1.2 for a more detailed etymological discussion. 2. In French translation, Méthode pour la Recherche du Maximum et du Minimum. Further quotations will be taken from the French text (1896). 3. Following the conventions established by Viète, Fermat uses capital letters of vowels A, E, I, O, U for variables, and capital letters of consonants for constants. 4. The notation “ϭAD” for adequality is ours, not Fermat’s. There is a variety of differ- ing notations in the literature; see Barner 2011 for a summary. 5. The use of the term “suppress” in reference to the remaining terms, rather than “set- ting them equal to zero,” is crucial; see n.33. 6. Note that division by e in Step 4 necessarily precedes the suppression of remaining terms that contain e in Step 6. Suppressing all terms containing e, or setting e equal to zero, at stage 3 would be meaningless; see n.22. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00101 by guest on 01 October 2021 Perspectives on Science 285 in powers of e and taking the constant term.7 The method (leaving aside step (5) for the moment) is immediately understandable to a modern reader as the elementary calculus exercise of ªnding the extremum by solving the equation fЈ(a) ϭ 0. But the real question is how Fermat un- derstood this algorithm in his own terms, in the mathematical language of his time, prior to the invention of calculus by Barrow, Leibniz, Newton, et al. There are two crucial points in trying to understand Fermat’s reason- ing: ªrst, the meaning of “adequality” in step (2), and second, the justi- ªcation for suppressing the terms involving positive powers of e in step (6). The two issues are closely related because interpretation of adequality depends on the conditions on e. One condition which Fermat always as- sumes is that e is positive. He did not use negative numbers in his calcula- tions. Fermat introduces the term adequality in Methodus with a reference to Diophantus of Alexandria. In the third article of the series, Ad Eamdem Methodum (Sur la Même Méthode), he quotes Diophantus’ Greek term â , which he translates following Xylander and Bachet, as adae- quatio or adaequalitas (1896, p. 126; see A. Weil 1984, p. 28). 1.2. Etymology of â . The Greek word â consists of the prepositional preªx para and the root isotes, “equality.” The preªx para, like all “regular” preªxes in Greek, also functions as a preposition indicating position; its basic meaning is that of proximity, but depending upon the construction in which it ap- pears, it can indicate location (“beside”), direction (“to”), or source (“from”) (see Luraghi 2003, pp. 20–22, 131–145). Compounds with all three meanings are found. Most familiar to mathematicians will be parallelos ( © ), used of lines that are “next to” one another; the Greek for “nearly resembling” is paraplesios ( Ô ); but we also ªnd direction in words like paradosis ( © ), “transmitting, handing over” (from para and dosis, “giving”), and source in words like paralepsis ( © ), “receiving” (from para and lepsis, “taking”); there are other meanings for this preªx that do not con- cern us. The combination of para and isos (“equal”) can refer either to a simple ~ equality (Aristotle, Rhetoric 1410a 23: parÉ ‰Ãª Ñ ª w , “It is parisosis if the members of the sentence are equal”) or an approximate equality (Strabo 11.7.1, ý ~h , Ò Ê © Ø ~i ã Á u~ Ú~ Ú~, “As Patrocles says, who also considers this sea [the Caspian] to be approximately equal [parison] to the Black Sea”). We know of no pas- 7. Fermat also envisions a division by a higher power of e as in step (5) (see Section 3). Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/POSC_a_00101 by guest on 01 October 2021 286 The Method of Adequality sage other than those of Diophantus in which a term involving para and isos refers to mathematical equality, whether approximate or otherwise. Diophantus himself used the term parisos to describe terms that are ap- proximately equal, as we shall demonstrate below (Subsection 8.1). The term isotes denotes a relationship (“equality”), not an action (“set- ting equal”); the normal term for the action of equalizing would be a form in -osis, and in fact the words isosis (Ñ ) and parisosis ( É ) are at- tested with the meaning “making equal” or “equalization”; the latter is a common term in rhetoric for using the same (or nearly the same) number of words or syllables in parallel clauses. The word â , which occurs only in the two Diophantus passages, is on the face of it more appropriate to the meaning “near equality.” Fermat himself may not have gotten this far into Greek etymology. On the other hand, Fermat viewed Diophantus through the lens of Bachet’s analysis. Bachet does interpret it as approximate equality. If Fermat follows Bachet, seeking to interpret Fermat’s method of adequality/ â on the basis of the Latin term adaequare misses the point (see Subsection 7.1). 1.3. Modern Interpretations There are differing interpretations of Fermat’s method in the literature. A. Weil notes that Diophantus uses the Greek term to designate his way of approximating a given number by a rational solution to a given problem. (cf. e.g. Dioph.V.11 and 14; Weil 1984, p. 28) According to Weil’s interpretation, approximation is implicit in the meaning of the original Greek term. H. Breger rejects Weil’s interpreta- tion of Diophantus, and proposes his own interpretation of the mathemat- ics of Diophantus’ â . He argues that â means equality to Diophantus (Breger 1994, p. 201). Thus, the question of whether there is an element of approximation in the Greek source of the term is itself sub- ject to dispute. There is also a purely algebraic aspect to adequality, based on the ideas of Pappus of Alexandria, and Fermat’s predecessor Viète.