1.NounIn operator theory, a set
X
⊆
C
{\displaystyle X\subseteq \mathbb {C} }
such that, for a (possibly unbounded) linear operator
T
{\displaystyle T}
on a Banach space, the spectrum of
T
{\displaystyle T}
is in
X
{\displaystyle X}
and von Neumann's inequality holds for
T
{\displaystyle T}
on
X
{\displaystyle X}
.