$n$-Extension closed subcategories of $n$-exangulated categories

Abstract: Let $n$ be a positive integer. We show that an $n$-extension closed subcategory of an $n$-exangulated category naturally inherits an $n$-exangulated structure through restriction of the ambient $n$-exangulated structure. Furthermore, we show that a strong version of the Obscure Axiom holds for $n$-exangulated categories, where $n \geq 2$. This allows us to characterize $n$-exact categories as $n$-exangulated categories with monic inflations and epic deflations. We also show that for an extriangulated category condition (WIC), which was introduced by Nakaoka and Palu, is equivalent to the underlying additive category being weakly idempotent complete. We then apply our results to show that $n$-extension closed subcategories of an $n$-exact category are again $n$-exact. Furthermore, we recover and improve results of Klapproth and Zhou.