  We investigate quantitative properties of nonnegative solutions $u(t,x)\ge 0$
to the nonlinear fractional diffusion equation, $\partial_t u +
\mathcal{L}F(u)=0$ posed in a bounded domain, $x\in\Omega\subset \mathbb{R}^N$,
with appropriate homogeneous Dirichlet boundary conditions. As $\mathcal{L}$ we
can use a quite general class of linear operators that includes the two most
common versions of the fractional Laplacian $(-\Delta)^s$, $0<s<1$, in a
bounded domain with zero Dirichlet boundary conditions, but it also includes
many other examples since our theory only needs some basic properties that are
typical of "linear heat semigroups." The nonlinearity $F$ is assumed to be
increasing and is allowed to be degenerate, the prototype is the power case
$F(u)=|u|^{m-1}u$, with $m>1$.
  In this paper we propose a suitable class of solutions of the equation, and
cover the basic theory: we prove existence, uniqueness of such solutions, and
we establish upper bounds of two forms (absolute bounds and smoothing effects),
as well as weighted-$L^1$ estimates. The class of solutions is very well suited
for that work. The standard Laplacian case $s=1$ is included and the linear
case $m=1$ can be recovered in the limit.
  In a companion paper [12], we will complete the study with more advanced
estimates, like the upper and lower boundary behaviour and Harnack
inequalities, for which the results of this paper are needed.
