Smoothed one-core and core--multi-shell regular black holes
  We discuss the generic properties of a general, smoothly varying, spherically
symmetric mass distribution $\mathcal{D}(r,\theta)$, with no cosmological term
($\theta$ is a length scale parameter). Observing these constraints, we show
that (a) the de Sitter behavior of spacetime at the origin is generic and
depends only on $\mathcal{D}(0,\theta)$, (b) the geometry may posses up to
$2(k+1)$ horizons depending solely on the total mass $M$ if the cumulative
distribution of $\mathcal{D}(r,\theta)$ has $2k+1$ inflection points, and (c)
no scalar invariant nor a thermodynamic entity diverges. We define new
two-parameter mathematical distributions mimicking Gaussian and step-like
functions and reduce to the Dirac distribution in the limit of vanishing
parameter $\theta$. We use these distributions to derive in closed forms
asymptotically flat, spherically symmetric, solutions that describe and model a
variety of physical and geometric entities ranging from noncommutative black
holes, quantum-corrected black holes to stars and dark matter halos for various
scaling values of $\theta$. We show that the mass-to-radius ratio $\pi c^2/G$
is an upper limit for regular-black-hole formation. Core--multi-shell and
multi-shell regular black holes are also derived.
