We solve the multiplicative Cauchy equation for real functions of symmetric positive definite matrices under the differentiability restriction. The specialty of the problem lies in the symmetry of the multiplication.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-ap82-1-1,
author = {Konstancja Bobecka and Jacek Weso\l owski},
title = {Multiplicative Cauchy functional equation in the cone of positive-definite symmetric matrices},
journal = {Annales Polonici Mathematici},
volume = {81},
year = {2003},
pages = {1-7},
zbl = {1099.39015},
language = {en},
url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ap82-1-1}
}
Konstancja Bobecka; Jacek Wesołowski. Multiplicative Cauchy functional equation in the cone of positive-definite symmetric matrices. Annales Polonici Mathematici, Tome 81 (2003) pp. 1-7. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ap82-1-1/