# An Operator Equation Generalizing the Leibniz Rule for the Second Derivative

Chapter
Part of the Lecture Notes in Mathematics book series (LNM, volume 2050)

## Abstract

We determine all operators $$T : {C}^{2}(\mathbb{R}) \rightarrow C(\mathbb{R})$$ and $$A : {C}^{1}(\mathbb{R}) \rightarrow C(\mathbb{R})$$ which satisfy the equation
$$T(f \cdot g) = (Tf) \cdot g + f \cdot (Tg) + (Af) \cdot (Ag)\ ;\quad f,g \in {C}^{2}(\mathbb{R}).$$
(1)
This operator equation models the second order Leibniz rule for (fg) with $$Af = \sqrt{2}f'$$. Under a mild regularity and non-degeneracy assumption on A, we show that the operators T and A have to be of a very restricted type. In addition to the operator solutions S of the Leibniz rule derivation equation corresponding to A = 0,
$$S(f \cdot g) = (Sf) \cdot g + f \cdot (Sg)\ ;\quad f,g \in {C}^{2}(\mathbb{R})\text{ or }{C}^{1}(\mathbb{R})\,$$
(2)
which are of the form
$$Sf = bf' + af\ln \vert f\vert,\quad a,b \in C(\mathbb{R}),$$
T and A may be of the following three types
$$\begin{array}{lll} Tf & = \frac{1} {2}{d}^{2}f'' &,\ Af = d\,f' \\ Tf & = \frac{1} {2}{d}^{2}f{(\ln \vert f\vert )}^{2} &,\ Af = d\,f\ln \vert f\vert \\ Tf & = {d}^{2}f(\epsilon \vert f{\vert }^{p} - 1)&,\ Af = d\,f(\epsilon \vert f{\vert }^{p} - 1) \end{array}$$
for suitable continuous functions d, c and p and where ε is either 1 or sgnf and p ≥ − 1. The last operator solution is degenerate in the sense that T is a multiple of A. We also determine all solutions of (1) if T and A operate only on positive $${C}^{2}(\mathbb{R})$$-functions or $${C}^{2}(\mathbb{R})$$-functions which are nowhere zero.

## Keywords

Functional Equation Additive Function Operator Equation Operator Solution Leibniz Rule
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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