zn + nzn−1 ov + ... = cn (xp + pxp−1 o + ...),
from which he deduced the relation
nzn−1 v = cn pxp−1
by omitting the equal terms zn and cnxp and dividing the remaining terms by o, tacitly putting o = 0 after division. This relation is the same as v = xm. Newton pointed out that, conversely, from the relation v = xm the relation z = xm+1 / (m + 1) follows. He applied his formula to the quadrature of curves whose ordinates can be expressed as the sum of a finite number of terms of the form axm; and gave examples of its application to curves in which the ordinate is expressed by an infinite series, using for this purpose the binomial theorem for negative and fractional exponents, that is to say, the expansion of (1 + x)n in an infinite series of powers of x. This theorem he had discovered; but he did not in this tract state it in a general form or give any proof of it. He pointed out, however, how it may be used for the solution of equations by means of infinite series. He observed also that all questions concerning lengths of curves, volumes enclosed by surfaces, and centres of gravity, can be formulated as problems of quadratures, and can thus be solved either in finite terms or by means of infinite series. In the Quadratura (1676) the method of integration which is founded upon the inversion-theorem was carried out systematically. Among other results there given is the quadrature of curves expressed by equations of the form y = xn (a + bxm)p; this has passed into text-books under the title “integration of binomial differentials” (see § 49). Newton announced the result in letters to Collins and Oldenburg of 1676.
21. In the Methodus fluxionum (1671) Newton introduced his characteristic notation. He regarded variable quantities as generated by the motion of a point, or line, or plane, and called the generated quantity a “fluent” and its rate of generation Newton’s method of Fluxions. a “fluxion.” The fluxion of a fluent x is represented by x, and its moment, or “infinitely” small increment accruing in an “infinitely” short time, is represented by ẋo. The problems of the calculus are stated to be (i.) to find the velocity at any time when the distance traversed is given; (ii.) to find the distance traversed when the velocity is given. The first of these leads to differentiation. In any rational equation containing x and y the expressions x + ẋo and y +ẏo are to be substituted for x and y, the resulting equation is to be divided by o, and afterwards o is to be omitted. In the case of irrational functions, or rational functions which are not integral, new variables are introduced in such a way as to make the equations contain rational integral terms only. Thus Newton’s rules of differentiation would be in our notation the rules (i.), (ii.), (v.) of § 11, together with the particular result which we write
| dxm | = mxm−1, (m integral). |
| dx |
a result which Newton obtained by expanding (x = ẋo)m by the binomial theorem. The second problem is the problem of integration, and Newton’s method for solving it was the method of series founded upon the particular result which we write
| ∫ xm dx = | xm+1 | . |
| m + 1 |
Newton added applications of his methods to maxima and minima, tangents and curvature. In a letter to Collins of date 1672 Newton stated that he had certain methods, and he described certain results which he had found by using them. These methods and results are those which are to be found in the Methodus fluxionum; but the letter makes no mention of fluxions and fluents or of the characteristic notation. The rule for tangents is said in the letter to be analogous to de Sluse’s, but to be applicable to equations that contain irrational terms.
22. Newton gave the fluxional notation also in the tract De Quadratura curvarum (1676), and he there added to it notation for the higher differential coefficients and for indefinite integrals, as we call them. Just as x, y, z, ... are fluents Publication of the Fluxional Notation. of which ẋ, ẏ, ̇z, ... are the fluxions, so ẋ, ẏ, ̇z, ... can be treated as fluents of which the fluxions may be denoted by ẍ, ̈y, ̈z,... In like manner the fluxions of these may be denoted by ẍ, ̈y, ̈z, ... and so on. Again x, y, z, ... may be regarded as fluxions of which the fluents may be denoted by ́x, ́y, ́z, ... and these again as fluxions of other quantities denoted by ̋x, ̋y, ̋z, ... and so on. No use was made of the notation ́x, ̋x, ... in the course of the tract. The first publication of the fluxional notation was made by Wallis in the second edition of his Algebra (1693) in the form of extracts from communications made to him by Newton in 1692. In this account of the method the symbols 0, ẋ, ẍ, ... occur, but not the symbols ́x, ̋x, .... Wallis’s treatise also contains Newton’s formulation of the problems of the calculus in the words Data aequatione fluentes quotcumque quantitates involvente fluxiones invenire et vice versa (“an equation containing any number of fluent quantities being given, to find their fluxions and vice versa”). In the Philosophiae naturalis principia mathematica (1687), commonly called the “Principia,” the words “fluxion” and “moment” occur in a lemma in the second book; but the notation which is characteristic of the calculus of fluxions is nowhere used.