Title: Singular Degenerations of Lie Supergroups of Type $D(2,1;a)$,
Abstract: The complex Lie superalgebras $\mathfrak{g}$ of type $D(2,1;a)$ - also
denoted by $\mathfrak{osp}(4,2;a) $ - are usually considered for "non-singular"
values of the parameter $a$, for which they are simple. In this paper we
introduce five suitable integral forms of $\mathfrak{g}$, that are well-defined
at singular values too, giving rise to "singular specializations" that are no
longer simple: this extends the family of simple objects of type $D(2,1;a)$ in
five different ways. The resulting five families coincide for general values of
$a$, but are different at "singular" ones: here they provide non-simple Lie
superalgebras, whose structure we describe explicitly. We also perform the
parallel construction for complex Lie supergroups and describe their singular
specializations (or "degenerations") at singular values of $a$. Although one
may work with a single complex parameter $a$, in order to stress the overall
$\mathfrak{S}_3$-symmetry of the whole situation, we shall work (following
Kaplansky) with a two-dimensional parameter $\boldsymbol{\sigma} =
(\sigma_1,\sigma_2,\sigma_3)$ ranging in the complex affine plane $\sigma_1 +
\sigma_2 + \sigma_3 = 0$.