Stability in Categories and Normal Projective Varieties over Perfect Fields
We present a notion of $Δ$-stability and stability filtration in arbitrary categories which is equivalent to the existence of Harder-Narasimhan (HN) sequences on objects. Indeed it is equivalent to the existence of a zero morphism, a partial order on objects, and a collection of some universal sequences. In additive categories embedded in an ambient triangulated category, we could obtain a numerical polynomial or central charge of objects by calculating the Euler characteristic of slope sequences and objects, inducing a partial order and HN sequences. In the case of normal projective surfaces over an arbitrary perfect field $k$ we show the existence of $Δ$-stabilities of degree one on the relevant bounded derived categories which is equivalent to the existence of Bridgeland's stabilities on normal surfaces. This result also leads to new effective restriction theorem of slope semistable sheave on normal projective varieties over perfect fields. Our approach also gives alternative proofs of Hodge Index Theorem and Bogomolov Inequality.