We explicitly compute a monoidal subcategory of the monoidal center of Deligne's interpolation category normalRe̲pfalse(Stfalse), for t not necessarily a natural number, and we show that this subcategory is a ribbon category. For t=n, a natural number, there exists a functor onto the braided monoidal category of modules over the Drinfeld double of Sn which is essentially surjective and full. Hence the new ribbon categories interpolate the categories of crossed modules over the symmetric groups. As an application, we obtain invariants of framed ribbon links which are polynomials in the interpolating variable t. These polynomials interpolate untwisted Dijkgraaf–Witten invariants of the symmetric groups.

We study Karoubian tensor categories, which interpolate representation categories of families of the so-called easy quantum groups in the same sense in which Deligne’s interpolation categories $\ensuremath {\mathop {\textrm {\underline {Rep}}}}(S_t)$ interpolate the representation categories of the symmetric groups. As such categories can be described using a graphical calculus of partitions, we call them interpolating partition categories. They include $\ensuremath {\mathop {\textrm {\underline {Rep}}}}(S_t)$ as a special case and can generally be viewed as subcategories of the latter. Focusing on semisimplicity and descriptions of the indecomposable objects, we prove uniform generalisations of results known for special cases, including $\ensuremath {\mathop {\textrm {\underline {Rep}}}}(S_t)$ or Temperley–Lieb categories. In particular, we identify those values of the interpolation parameter, which correspond to semisimple and non-semisimple categories, respectively, for all the so-called group-theoretical easy quantum groups. A crucial ingredient is an abstract analysis of certain subobject lattices developed by Knop, which we adapt to categories of partitions. We go on to prove a parametrisation of the indecomposable objects in all interpolating partition categories for non-zero interpolation parameters via a system of finite groups, which we associate to any partition category, and which we also use to describe the associated graded rings of the Grothendieck rings of these interpolation categories.

We classify the indecomposable objects in the monoidal center of Deligne's interpolation category Rep 𝑆 𝑡 by viewing Rep 𝑆 𝑡 as a model-theoretic limit in rank and characteristic. We further prove that the center of Rep 𝑆 𝑡 is semisimple if and only if 𝑡 is not a non-negative integer. In addition, we identify the associated graded Grothendieck ring of this monoidal center with that of the graded sum of the centers of representation categories of finite symmetric groups with an induction product. We prove analogous statements for the abelian envelope.

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