Some thoughts on differential equation notation - functional vs classical

The idea

is a nice alternative to

The idea: modern functional notation for derivatives is a nice alternative to the commonly used classical notation. I find it is easier to understand, less ambiguous, lends itself to execution, and has less tendency to motivate little deceits.

CREDITS: Most of these ideas come from an old draft of "Variational Mechanics: a Functional Approach" (as yet unpublished?), by Harold Abelson, Meinhard Mayer, Gerald Sussman, and Jack Wisdom. The idea for the extended example below comes, via them (page vi), from Michael Spivak's Calculus on Manifolds (1965).
[Update 2001 - The book is now available as the Structure and Interpretation of Classical Mechanics! ]
The mistakes are my contribution.
I hope they will forgive me for lifting a particularly fun quote (from the Preface, page v):

"In almost all textbooks, even the best, this principle is presented so that it is impossible to understand." (K. Jacobi Lectures on Dynamics, 1842-1843). I have not chosen to break with tradition.
V.I. Arnold, Mathematical Methods of Classical Mechanics (1980), footnote on p.246

Contents

A side by side look at various expressions
Some problems with the classical notation
An equation worked in both notations
Executing the functional notation (an almost empty section)

Side by side

 classical: functional: meaning: a function, f, of one variable or the value of f when it is evaluated at a the derivative of f, which is itself a function! It takes the same arguments as f does. the value of the derivative of f, when it is evaluated at 3 another function a function, the sum of two functions which take the same arguments the value of when it is evaluated at 3 (namely the sum of the values of f and g when both are evaluated at 3) a function, the product of two functions which take the same arguments the value of when it is evaluated at 3 (namely the product of the values of f and g when both are evaluated at 3) a function, the composition of f and g (ie, f is called on the result of g called on the argument to the new function). It thus takes the same arguments as g. another function, the composition of f and the derivative of g (ie, f is called on the result of Dg called on the argument to the new function) another function the partial derivative of h with respect to its 1st argument the value of when evaluated at (4,5) (namely, the partial derivative of h, in the neighborhood of (4,5), with respect to its first argument) the composition of f and h the partial derivative of with respect to its 1st argument. a function (of course), which gives the derivative of f at values given by h the derivative of f at the value of h evaluated at (4,5) hmm... what do you think?

Some problems with the classical notation

• Obscurity creates teaching difficulties, such as the alleged need to explain to beginning students why you can't evaluate at 3 (ie (Df)(3)) by substituting 3 for x, (producing nonsense).

• Which is clearer? The chain rule expressed as
,
or as
.
• But more fundamentally, classical cumbersomeness motivates pathologies. A verbose equation needs to be shrunk. To shrink an equation, content is stripped out and scattered to the surrounding text. To gain some conciseness, ambiguity is used, with similar expressions meaning different things in different parts of an equation. This can then corrupt the associated reasoning (ie, there are allegedly a number of derivations of Hamilton's equations, which contain a step asserting "and now we consider the momentum to be independent of the velocities", an obvious absurdity, apparently motivated by notational opacity).

An equation in two notations

Lets dissect an classical example, and then compare the functional equivalent.

where f(u,v) is a function of u=g(x,y) and v=h(x,y).

For ambiguity, lets see what three seemingly similar subexpressions mean ( , , ):
 means the partial derivative of f with respect to its first argument u means means
So can you tell what an equation expression means just by looking at it? No. To disambiguate it one has to play detective and examine the variables and the surrounding text.

In contrast, here is the functional version.

The meaning of this equation can be determined systematically from the meaning of its parts.

is a function. It takes the same arguments as g and h take (thus g and h must both be taking the same kind of arguments). f itself takes two arguments, one from g and one from h. The function gives the values f gives, when f is given the values g and h give, when both given same arguments. Expressed differently: there are some arguments. g and h independently chew on them. Their two results are given to f, which then provides the final result.

is a function. It is the derivative of with respect to one of its arguments, x. The function's arguments are the same as those of , and . In english, it is the rate of change in the values f gives (f's arguments having been provided by g and h from their arguments), in the neighborhood specified by those same arguments, as one of those arguments (x) varies.

Perhaps expressed more clearly... g and h take identical arguments. Those arguments define a point in space. g and h each provide some value for that point. f computes its own value for that point, computing it from the values given by g and h. As one of those arguments, x, varies, jiggling the point, the value of f also jiggles. How fast f jiggles relative to the argument's jiggling is the subject of this equation. is just a way of saying this.

As for the right hand side...

is a function. It takes the same arguments as f does. Its the partial derivatives of f with respect to its first argument. This is how fast f jiggles, in general, compared to its first argument's jiggling.

is a function. It takes the same arguments as g and h do. Its values are the values the function gives, when given the values that g and h give, when given those arguments. This is how fast f jiggles compared to g's jiggling, at the specific point described by the arguments. (The specific point matters because, for instance, the value of h might affect f's jiggling, and h could give different values for different arguments.)

is a function. It takes the same arguments as g does. Its the partial derivatives of g with respect to its argument x. It is how fast g jiggles with respect to jiggles in x.
is a function. It takes the same arguments as g and h do. Its values are the product of the values given by and when these are given those same arguments. It is how fast f jiggles given g's jiggling, times how fast g jiggles given x's jiggles. All for some particular bunch of arguments.

is a function. Its derivation and meaning are similar. It is how fast f jiggles given h's jiggling, times how fast h jiggles given x's jiggles. All for some particular bunch of arguments.

Finally,
is a function. It takes the same arguments as g and h do. Its values are the sum of the values given by and when these are given those same arguments. It is the sum of f's jiggling induced by g's jiggling, plus f's jiggling induced by h's jiggling.

In summary, the right hand side of the equation, , tells how to calculate this jiggling in f, caused via g and h, by the jiggles of their argument x. It is the sum of two contributors. The first is how fast f jiggles when g is jiggled by x, and the second is how fast f jiggles when h is jiggled. The Aha! neatness of this equation is that the contributions via g and via h are independent of each other. One could imagine this not being the case - there could have been some weird interrelationship between the two contributions. The insight is that the contribution of g's jiggling to f's jiggling is not affected by h's jiggling, and visa versa. They are independent, and thus can be computed separately and then simply added together.

Now you can perhaps go back, look at the classical notation, and see the meaning there. This ability to more or less understand the classical notation once you know what you are looking for, are what allow it to be used at all. The necessity, to you know what you are looking for, is what makes the classical notation such a bane for students, and trap for professional users.

Execution, and the functional notation

[This section is basically just a placeholder.]
The functional version of the above equation can be straightforwardly converted into a form which can be executed.
Rather loosely, in a dialect of Scheme:
```(define (indirect-jiggling-function f g h x)
(+ (* ((partial x) g)
(((partial 1) f) g h))
(* ((partial x) h)
(((partial 2) f) g h))))
```

More credits:
The equation images where created using Nikos Drakos's LaTeX2HTML.

```Doables: