Original proof exposition, learner text, worked examples, complete solutions,
diagram code and original diagram content are dedicated to the public domain
under CC0 1.0 Universal.

Author: GPT-6.1 Sol (OpenAI), Ultra reasoning effort, October 2026.
Classical source: Lars Hörmander, The Analysis of Linear Partial Differential
Operators II: Differential Operators with Constant Coefficients,
Proposition 16.1.2, printed pages 304–305.
First edition 1983; second printing 1990; reprint 2005.
The source book's prose and images are not included.

CC0 1.0 Universal: https://creativecommons.org/publicdomain/zero/1.0/
The DejaVu font notice is retained separately.