If k is a field, A a finite dimensional k-algebra, then the simple A-modules form a simple minded collection in the derived category Db(modA)\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {D}^{ {\text {b}}}( {\text {mod}}\,A )$$\end{document}. Their extension closure is modA\documentclass[12pt]{minimal}
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\begin{document}$${\text {mod}}\,A$$\end{document}; in particular, it is abelian. This situation is emulated by a general simple minded collection S\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {S}$$\end{document} in a suitable triangulated category C\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {C}$$\end{document}. In particular, the extension closure ⟨S⟩\documentclass[12pt]{minimal}
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\begin{document}$$\langle \mathscr {S}\rangle $$\end{document} is abelian, and there is a tilting theory for such abelian subcategories of C\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {C}$$\end{document}. These statements follow from ⟨S⟩\documentclass[12pt]{minimal}
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\begin{document}$$\langle \mathscr {S}\rangle $$\end{document} being the heart of a bounded t-structure. It is a defining characteristic of simple minded collections that their negative self extensions vanish in every degree. Relaxing this to vanishing in degrees {-w+1,…,-1}\documentclass[12pt]{minimal}
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\begin{document}$$\{ -w+1, \ldots , -1 \}$$\end{document} where w is a positive integer leads to the rich, parallel notion of w-simple minded systems, which have recently been the subject of vigorous interest. If S\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {S}$$\end{document} is a w-simple minded system for some w⩾2\documentclass[12pt]{minimal}
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\begin{document}$$w \geqslant 2$$\end{document}, then ⟨S⟩\documentclass[12pt]{minimal}
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\begin{document}$$\langle \mathscr {S}\rangle $$\end{document} is typically not the heart of a t-structure. Nevertheless, using different methods, we will prove that ⟨S⟩\documentclass[12pt]{minimal}
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\begin{document}$$\langle \mathscr {S}\rangle $$\end{document} is abelian and that there is a tilting theory for such abelian subcategories. Our theory is based on Quillen’s notion of exact categories, in particular a theorem by Dyer which provides exact subcategories of triangulated categories. The theory of simple minded systems can be viewed as “negative cluster tilting theory”. In particular, the result that ⟨S⟩\documentclass[12pt]{minimal}
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\begin{document}$$\langle \mathscr {S}\rangle $$\end{document} is an abelian subcategory is a negative counterpart to the result from (higher) positive cluster tilting theory that if T\documentclass[12pt]{minimal}
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\begin{document}$$\mathscr {T}$$\end{document} is a cluster tilting subcategory, then (T∗ΣT)/[T]\documentclass[12pt]{minimal}
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\begin{document}$$( \mathscr {T}* \Sigma \mathscr {T})/[ \mathscr {T}]$$\end{document} is an abelian quotient category.
