being equal; and similarly (d) dy" represents the mth dif dym dyTM ferential of u, formed by making y to increase m times succes sively by equal increments dy; and (; dru dxm dyr ;) dæm dyr-m indi cates the rth differential of u, formed by making a to increase m times successively by equal increments da, and y to increase (r-m) times successively by equal increments dy. The principles on which this notation is constructed are so immediately applicable to functions of more than two variables, that it is unnecessary to do more than state the results as they occur in the following Articles. Here however I must remark on a defect in the notation. If u = F(x, y), for instance, the same symbol d'u in the numerators of (dzu), dx2 d2u dx dy d2u dy2 means processes which, although of the same kind, may lead to results altogether different. In the first it means the second differential of u springing from two successive variations of x; in the second it represents d'u springing from a variation of x, taking place on the back of a previous variation of y; and in the third, d'u as originating in two successive variations of y. The brackets therefore are used in conjunction with the different denominators to indicate these variations of the same symbol, and are marks sufficiently distinctive to prevent confusion. But before we proceed further, we must prove the following proposition. 79.] If a function is differentiated many times in respect of independent variables which it contains, the result is the same, whatever is the order of the variables with respect to which it is differentiated: provided that it is differentiated the same number of times and with respect to the same variables. For the sake of convenience, using symbols of differentiation, dxF (x, y, z,...) = F(x+dx, y, z, ...) — F (X, Y, Z, ...), dyr (x, y, z,...) = F(x, y + dy, z, ...) — F (X, Y, Z, ...) ; (93) (94) therefore from (93), dyd, F(x, y, z, ...) = F(x + dx, y + dy, z, ...) — F (x+dx, y, z, ...) -F(x, y+dy, z,...) + F(x, y, z,...); and from (94), dx d1 F(x, y, z, ...) = F(x+dx, y + dy, z,...) — F(x, y+dy, z,...) F(x + dx, y, z, ...) + F(X, Y, Z,...); and as the two results are identical, it is manifest that dx dy r(x, y, z,...) = dyd, F(x, y, z,...); or, writing the result according to the notation of Art. 46, (95) As the proof here given does not depend on the differentiations having been performed with respect to two variables only, it is plain that an analogous theorem is true for differentiations with respect to any number of variables; so that we may always interchange, in whatever manner it is convenient, the order in which the several differentiations are performed; as, for instance, Hence also it follows, that if successive partial differentiations are performed on a function of many independent variables, by making x, y, and z to vary severally r times, s times, and t times, the order of these variations may be interchanged in any permutation, and the result is the same; thus if Of this property, thus proved in the general case, some par ticular examples are subjoined. 80.] Differentiation of a function of two independent variables. D (1) dx + D (du) dy + (de) d2x + (du) d2y. .. D.DU D dy du dy are functions of x and y; so du d d dx dx dx + dy, dx dy = (day) dx dx2 du (du d dy dy dx + dy, dx dy dy whence, taking the most general case, in which neither a nor y is equicrescent, we have the brackets indicating that the derived-functions within them Similarly may other total differentials be found; but the general term is too complicated to be of any practical use. Now let the results be simplified by making x and y equicrescent, neither of which assumptions is inconsistent with the given equation; then and so on; the law of the coefficients being the same as that of (1+a)"; which may be proved to be true for positive integral values of the exponent, by a train of reasoning similar to that in Art. 55; whence the nth differential is ... D2u = {a2 dx2+2 ab dx dy + b2 dy2} eax+by, 81.] By a similar process, if u is a function of many independent variables, x, y, z, all of which are equicrescent, we have the following series of equations : u = F(X, Y, Z, .......), ... dy2 + (d2u) dz2 + (d2) dx2 + (d2) dy2 + ( d2u dz2 d2u ...... (105) + 2 (dd) dy dz + 2 ( 1211 ) dz dx + 2 (dx dy) dx dy + ... (106) dy dz If x, y, z, ...... dz dx are not equicrescent, terms will have to be added analogous to those which, by reason of x and y being equicrescent, vanished from equations (100) and (101). |