Two subfactors arising from a non-degenerate commuting square: An answer to a question raised by V.F.R. Jones

When we have a non-degenerate commuting square of finite dimensional C∗-algebras, we can construct a subfactor in two ways. One is by a repetition of basic constructions in a horizontal direction and the other in a vertical direction. We prove that if one of the two is of finite depth, so is the other. Furthermore, we prove the two have the same global indices in the sense of A. Ocneanu. This gives an answer to a question V.F.R. Jones raised in his talk at Aarhus in June, 1995. We actually prove a more general result on flatness and also give an example of a new finite principal graph as its application.