Paper detail

On the equality of operator valued weights

G. K. Pedersen and M. Takesaki have proved in 1973 that if $φ$ is a faithful, semi-finite, normal weight on a von Neumann algebra $M\;\!$, and $ψ$ is a $σ^φ$-invariant, semi-finite, normal weight on $M\;\!$, equal to $φ$ on the positive part of a weak${}^*$-dense $σ^φ$-invariant $*$-subalgebra of $\mathfrak{M}_φ\;\!$, then $ψ=φ\;\!$. In 1978 L. Zsidó extended the above result by proving: if $φ$ is as above, $a\geq 0$ belongs to the centralizer $M^φ$ of $φ\;\!$, and $ψ$ is a $σ^φ$-invariant, semi-finite, normal weight on $M\;\!$, equal to $φ_a:=φ(a^{1/2}\;\!\cdot\;\! a^{1/2})$ on the positive part of a weak${}^*$-dense $σ^φ$-invariant $*$-subalgebra of $\mathfrak{M}_φ\;\!$, then $ψ=φ_a\;\!$. Here we will further extend this latter result, proving criteria for both the inequality $ψ\leqφ_a$ and the equality $ψ=φ_a\;\!$. Particular attention is accorded to criteria with no commutation assumption between $φ$ and $ψ\;\!$, in order to be used to prove inequality and equality criteria for operator valued weights. Concerning operator valued weights, it is proved that if $E_1\;\! ,E_2$ are semi-finite, normal operator valued weights from a von Neumann algebra $M$ to a von Neumann subalgebra $N\ni 1_M$ and they are equal on $\mathfrak{M}_{E_1}\;\!$, then $E_2\leq E_1\;\!$. Moreover, it is shown that this happens if and only if for any (or, if $E_1\;\! ,E_2$ have equal supports, for some) faithful, semi-finite, normal weight $θ$ on $N$ the weights $θ\circ E_2\;\! ,θ\circ E_1$ coincide on $\mathfrak{M}_{θ\circ E_1}\;\!$.