Some remarks on tangent martingale difference sequences in $L^1$-spaces

Let X be a Banach space. Suppose that for all $p\in (1,\mathrm{\infty })$ a constant $C_{p,X}$ depending only on X and p exists such that for any two X-valued martingales f and g with tangent martingale difference sequences one has $$\text{E}\mathrm{\parallel }f{\mathrm{\parallel }}^p\le C_{p,X}\text{E}\mathrm{\parallel }g{\mathrm{\parallel }}^p(\ast )\mathrm{.}$$ This property is equivalent to the UMD condition. In fact, it is still equivalent to the UMD condition if in addition one demands that either f or g satisfy the so-called (CI) condition. However, for some applications it suffices to assume that () holds whenever g satisfies the (CI) condition. We show that the class of Banach spaces for which () holds whenever only g satisfies the (CI) condition is more general than the class of UMD spaces, in particular it includes the space L^1. We state several problems related to (*) and other decoupling inequalities.