= Rees ring of a filtered algebra
{c}
{title2=$\mathcal R_F(A)=\bigoplus_{m\ge0}F_mA\,h^m\subseteq A[h]$}
For an ascending multiplicative filtration with $1\in F_0A$, its Rees ring contains the central element $h$. The quotient by $h$ is the <associated graded ring>, and the quotient by $h-1$ is the original algebra. If $[F_iA,F_jA]\subseteq F_{i+j-1}A$, commutators of lifts are divisible by $h$. Their first-order term defines the <Poisson bracket from a central deformation parameter> on the associated graded ring.
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