I was reading the paper, which you can find: https://hal.inria.fr/inria-00567668/document. I got so confused with equation (33) and (34), it seems like instead of an equal sign in eq.(33) on the left that should be a multiplication, which can be supported by the first author's thesis: https://pastel.archives-ouvertes.fr/pastel-01038025/document, eq.(2.62).
But I cannot understand what it means physically, can someone help me understand this!