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Singular Degenerations of Lie Supergroups of Type $D(2,1;a)$
Symmetry, integrability and geometry: methods and applications, Tome 14 (2018) Cet article a éte moissonné depuis la source Math-Net.Ru

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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 $\sigma = (\sigma_1,\sigma_2,\sigma_3)$ ranging in the complex affine plane $\sigma_1 + \sigma_2 + \sigma_3 = 0$.
Keywords: Lie superalgebras; Lie supergroups; singular degenerations; contractions. 
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     author = {Kenji Iohara and Fabio Gavarini},
     title = {Singular {Degenerations} of {Lie} {Supergroups} of {Type} $D(2,1;a)$},
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Kenji Iohara; Fabio Gavarini. Singular Degenerations of Lie Supergroups of Type $D(2,1;a)$. Symmetry, integrability and geometry: methods and applications, Tome 14 (2018). http://geodesic.mathdoc.fr/item/SIGMA_2018_14_a136/

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