Numerical Ranges in II$_1$ Factors

In this paper, we generalize the notion of the $C$-numerical range of a matrix to operators in arbitrary tracial von Neumann algebras. For each self-adjoint operator $C$, the $C$-numerical range of such an operator is defined; it is a compact, convex subset of $\mathbb{C}$. We explicitly describe the $C$-numerical ranges of several operators and classes of operators.