show/hide this revision's text 2 Typo in the condition

The necessary and sufficient condition is that $|x|^p$ and $|y|^q$ |y^*|^q$ are proportional. This can be deduced from Dixmier's paper (although it is not clearly stated that way there; it is based on Proposition 8). It probably also appears in more modern treatments (Nelson, Terp, Haagerup, Hiai, Kosaki, etc.) but I don't have the sources here to check that.

show/hide this revision's text 1

The necessary and sufficient condition is that $|x|^p$ and $|y|^q$ are proportional. This can be deduced from Dixmier's paper (although it is not clearly stated that way there; it is based on Proposition 8). It probably also appears in more modern treatments (Nelson, Terp, Haagerup, Hiai, Kosaki, etc.) but I don't have the sources here to check that.