A mathematical Coulomb branch for a complex reductive group
and a representation
is the spectrum of the commutative Braverman-Finkelberg-Nakajima convolution algebra.
Let
,
, and
The equivalence is
for
.
For a commuting torus
,
its parameter-deformed version is
, with
and convolution multiplication. This
is the commutative algebra, without the loop-rotation
parameter of the quantized version.
For quiver data , the topological grading is by
. It defines an algebraic
torus with character lattice
. A stability character specifies a cocharacter and its
scheme-theoretic fixed locus for the Hikita comparison.