  In this short article, given a smooth diagonalizable group scheme G of finite
type acting on a smooth quasi-compact quasi-separated scheme X, we prove that
(after inverting some elements of representation ring of G) all the information
concerning the additive invariants of the quotient stack [X/G] is
"concentrated" in the subscheme of G-fixed points X^G. Moreover, in the
particular case where G is connected and the action has finite stabilizers, we
compute the additive invariants of [X/G] using solely the subgroups of roots of
unity of G. As an application, we establish a Lefschtez-Riemann-Roch formula
for the computation of the additive invariants of proper push-forwards.
