  In this paper we prove a formula relating the equivariant Euler
characteristic of $K$-theoretic stable envelopes to an object known as the
index vertex for the cotangent bundle of the full flag variety. Our formula
demonstrates that the index vertex is the power series expansion of a rational
function. This result is a consequence of the 3d mirror self-symmetry of the
variety considered here. In general, one expects an analogous result to hold
for any two varieties related by 3d mirror symmetry.
