It is shown that commutativity of two oblique projectors is equivalent with their product idempotency if both projectors are not necessarily Hermitian but orthogonal with respect to the same inner product.
@article{bwmeta1.element.bwnjournal-article-doi-10_7151_dmps_1098, author = {Rados\l aw Kala}, title = {On commutativity of projectors}, journal = {Discussiones Mathematicae Probability and Statistics}, volume = {28}, year = {2008}, pages = {157-165}, zbl = {1158.15012}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_7151_dmps_1098} }
Radosław Kala. On commutativity of projectors. Discussiones Mathematicae Probability and Statistics, Tome 28 (2008) pp. 157-165. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_7151_dmps_1098/
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