We introduce a class of BMO spaces which interpolate with
and are
sufficiently large to serve as endpoints for new singular integral operators. More precisely,
let
be a
-finite
measure space. Consider two filtrations of
by successive refinement
of two atomic
-algebras
and
having trivial
intersection. Construct the corresponding truncated martingale BMO spaces. Then, the
intersection seminorm only leaves out constants and we provide a quite flexible condition on
so that the resulting
space interpolates with
in the expected way. In the presence of a metric
,
we obtain endpoint estimates for Calderón–Zygmund operators on
under additional
conditions on
.
These are weak forms of the “isoperimetric” and the “locally doubling” properties
of Carbonaro, Mauceri and Meda which admit less concentration at the
boundary. Examples of particular interest include densities of the form
for
any
or
for
any
. A
(limited) comparison with Tolsa’s RBMO is also possible. On the other hand, a more
intrinsic formulation yields a Calderón–Zygmund theory adapted to regular filtrations
over
without using a metric. This generalizes well-known estimates for perfect dyadic and
Haar shift operators. In contrast to previous approaches, ours extends to
matrix-valued functions (via recent results from noncommutative martingale theory)
for which only limited results are known and no satisfactory nondoubling theory
exists so far.