The triple product rule, known variously as the cyclic chain rule, cyclic relation, cyclical rule or Euler's chain rule, is a formula which relates partial derivatives of three interdependent variables. The rule finds application in thermodynamics, where frequently three variables can be related by a function of the form f(x, y, z) = 0, so each variable is given as an implicit function of the other two variables. For example, an equation of state for a fluid relates temperature, pressure, and volume in this manner. The triple product rule for such interrelated variables x, y, and z comes from using a reciprocity relation on the result of the implicit function theorem and is given by
Here the subscripts indicate which variables are held constant when the partial derivative is taken. That is, to explicitly compute the partial derivative of x with respect to y with z held constant, one would write x as a function of y and z and take the partial derivative of this function with respect to y only.
The advantage of the triple product rule is that by rearranging terms, one can derive a number of substitution identities which allow one to replace partial derivatives which are difficult to analytically evaluate, experimentally measure, or integrate with quotients of partial derivatives which are easier to work with. For example,
Various other forms of the rule are present in the literature; these can be derived by permuting the variables {x, y, z}.
An informal derivation follows. Suppose that f(x, y, z) = 0. Write z as a function of x and y. Thus the total differential dz is
Suppose that we move along a curve with dz = 0, where the curve is parameterized by x. Thus y can be written in terms of x, so on this curve
Therefore, the equation for dz = 0 becomes
Since this must be true for all dx, rearranging terms gives
Dividing by the derivatives on the right hand side gives the triple product rule
Note that this proof makes many implicit assumptions regarding the existence of partial derivatives, the existence of the exact differential dz, the ability to construct a curve in some neighborhood with dz = 0, and the nonzero value of partial derivatives and their reciprocals. A formal proof based on mathematical analysis would eliminate these potential ambiguities.
Suppose a function f(x,y,z)=0, where x,y and z are functions of each other. Write the total differentials of the variables
Substitute dy into dx
By using the chain rule one can show the coefficient of dx is equal to one, thus the coefficient of dz must be zero
Subtracting the second term and multiplying by its inverse gives the triple product rule
A geometric realization of the triple product rule can be found in its close ties to the velocity of a traveling wave
shown on the right at time t (solid blue line) and at a short time later t+?t (dashed). The wave maintains its shape as it propagates, so that a point at position x at time t will correspond to a point at position x+?x at time t+?t,
This equation can only be satisfied for all x and t if k?x-t=0, resulting in the formula for the phase velocity
To elucidate the connection with the triple product rule, consider the point p_{1} at time t and its corresponding point (with the same height) p?_{1} at t+?t. Define p_{2} as the point at time t whose x-coordinate matches that of p?_{1}, and define p?_{2} to be the corresponding point of p_{2} as shown in the figure on the right. The distance ?x between p_{1} and p?_{1} is the same as the distance between p_{2} and p?_{2} (green lines), and dividing this distance by ?t yields the speed of the wave.
To compute ?x, consider the two partial derivatives computed at p_{2},
Dividing these two partial derivatives and using the definition of the slope (rise divided by run) gives us the desired formula for
where the negative sign accounts for the fact that p_{1} lies behind p_{2} relative to the wave's motion. Thus, the wave's velocity is given by
For infinitesimal ?t, and we recover the triple product rule