Linear ODEs – What Do We Learn from the Wronskian?

ca.classical-analysis-and-odesdifferential equationsteaching

For a real interval $I$ and a continuous function $A: I \to \mathbb{R}^{d\times d}$, let $(x_1, \dots, x_d)$ denote a basis of the solution space of the non-autonomous ODE
$$
\dot x(t) = A(t) x(t) \quad \text{for} \quad t \in I.
$$
The mapping
$$
\varphi: I \ni t \mapsto \det(x_1(t), \dots, x_d(t)) \in \mathbb{R}
$$
is usually called the Wronskian of the basis $(x_1,\dots,x_d)$, and it seems to be an obligatory topic in every ODE course or book that I've seen.

So in an ODE course that I am currently teaching, I'm facing the following problem:

(1) Despite its prevalence in courses and textbooks, I've rarely (not to say never) encountered any situation where the Wronskian of an ODE is used in a way that sheds non-trivial insight onto the problem at hand – in particular not in any of the books where I've read about it. (Of course, I have also searched on the internet for it, but without any success.)

(2) I feel quite uneasy to teach a concept which I am unable to motivate properly.

(3) I'd feel even more uneasy to just omit it from the course, since chances are that my not knowing of an application of the Wronskian is just due to my ignorance.

Well, what I did is to merely mention the Wronskian in a remark – but of course (and fortunately) I did not get away with it, because quite soon a student asked what the Wronskian is good for.

So this is the

Question: What is the Wronskian (in the context of linear ODEs) good for?

Remarks.

  • One can show that $\varphi$ satisfies the differential equation
    $$
    \dot \varphi(t) = \operatorname{tr}(A(t)) \varphi(t),
    $$
    and since this a one-dimensional equation we have the solution formula
    $$
    (*) \qquad \dot \varphi(t) = e^{\int_{t_0}^t \operatorname{tr}(A(s)) \, ds} \varphi(t_0)
    $$
    for it (for any fixed time $t_0$ and all $t \in I$). This is nice – but still I can't see how to explain to my students that it is useful.

  • I've often seen discussions to the end that $(*)$ implies that "the Wronskian is non-zero at a time $t_0$ if and only if it is non-zero at every time $t$" – but I find this somewhat straw man-ish: the fact that $(x_1(t), \dots, x_d(t))$ is linearly independent at one time $t_0$ if and only if it is linearly independent at every time $t$ is an immediate consequence of the uniqueness theorem for ODEs, without any reference to the Wronskian.

  • One can give a geometrical interpretation of $(*)$: For instance, if all the matrices $A(t)$ have trace $0$, and it follows that the (non-autonomous) flow associated with our differential equation is volume preserving. However, I'm not convinced that this serves as a sufficient motivation to give the mapping $t \mapsto \det(x_1(t), \dots, x_d(t))$ its own name and to discuss it in some detail.

  • Maybe a word on the notion "good for" that occurs in the question: I'm pretty comfortable with studying and teaching mathematical objects just in order to better understand them, or for the sake of their intrinsic beauty. However, whenever we do so, this usually happens within a certain theoretical context – i.e., we build a theory, introduce terminology, and this terminology somehow contributes to the development (or to our understanding) of the theory.

    Some my question could be rephrased as:

    "I'm looking either (i) for applications of the Wronskian of ODEs to concrete problems (within or without mathematics) or (ii) for ways in which the concept 'Wronskian' facilitates our understanding of the theory of ODEs (or of any other theory)."

  • The term 'Wronskian' also seems to be used with a more general meaning (see for instance this Wikipedia entry). However, I am specifically interested in the Wronskian for the solutions of a linear ODE.

Best Answer

Here is a typical use in an undergraduate textbook: to prove that for distinct $\lambda_j$ the exponentials $e^{\lambda_jt}$ are linearly independent. It has some applications on the more advanced level, but you were asking about undergraduate textbooks. Also notice: uniqueness theorem, even for linear ODE is rarely proved in undergraduate textbooks, at least in the USA. So for linear equations with constant coefficients, the notion of Wronskian permits you to find $n$ linearly independent solutions without an appeal to the unproved uniqueness theorem. Same applies to the proof that cosines with distinct frequencies are linearly independent.

Another application. How to write a linear differential equation of order $n$ satisfied by $n$ given functions $f_1,\ldots,f_n$? Here is how: $$\left|\begin{array}{cccc}w&f_1&\ldots&f_n\\ w'&f_1^\prime&\ldots&f_n^\prime\\ \ldots&\ldots&\ldots&\ldots\\ w^{(n)}&f_1^{(n)}&\ldots&f_n^{(n)}\end{array}\right|=0.$$ Expanding with respect to the first column, we obtain that the Wronskian $W=W(f_1,\ldots,f_n)$ is the coefficient at $w^{(n)}$, in particular, if all $f_j$ are analytic then the singular points of the equation are the zeros of $W$.

The significance of the Wronskian is not limited to differential equations. Consider a finite-dimensional vector space $V$ consisting of functions. (For example, polynomials of degree at most $n$). Suppose we have a basis $f_1,\ldots,f_n$. How to expand a function $f\in V$ in this basis? Write $$f=c_1f_1+\ldots+c_nf_n,$$ differentiate $n-1$ times and solve the linear system with respect to $c_j$. The determinant of this system is the Wronskian. This was the original goal of Heine-Wronski when he invented it.

For less elementary applications, type "Wronski map" in the cell "Anywhere" or in the cell "Title" in Mathscinet search.

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