Bridgeland stability condition

The definitions in this section are presented as in the original paper of Bridgeland, for arbitrary triangulated categories.[2] Let ${\displaystyle {\mathcal {D}}}$ be a triangulated category. A slicing ${\displaystyle {\mathcal {P}}}$ of ${\displaystyle {\mathcal {D}}}$ is a collection of full additive subcategories ${\displaystyle {\mathcal {P}}(\varphi )}$ for each ${\displaystyle \varphi \in \mathbb {R} }$ such that

• ${\displaystyle {\mathcal {P}}(\varphi )[1]={\mathcal {P}}(\varphi +1)}$ for all ${\displaystyle \varphi }$, where ${\displaystyle [1]}$ is the shift functor on the triangulated category,
• if ${\displaystyle \varphi _{1}>\varphi _{2}}$ and ${\displaystyle A\in {\mathcal {P}}(\varphi _{1})}$ and ${\displaystyle B\in {\mathcal {P}}(\varphi _{2})}$, then ${\displaystyle \operatorname {Hom} (A,B)=0}$, and
• for every object ${\displaystyle E\in {\mathcal {D}}}$ there exists a finite sequence of real numbers ${\displaystyle \varphi _{1}>\varphi _{2}>\cdots >\varphi _{n}}$ and a collection of triangles

with ${\displaystyle A_{i}\in {\mathcal {P}}(\varphi _{i})}$ for all ${\displaystyle i}$.

The last property should be viewed as axiomatically imposing the existence of Harder–Narasimhan filtrations on elements of the category ${\displaystyle {\mathcal {D}}}$.

A Bridgeland stability condition on a triangulated category ${\displaystyle {\mathcal {D}}}$ is a pair ${\displaystyle (Z,{\mathcal {P}})}$ consisting of a slicing ${\displaystyle {\mathcal {P}}}$ and a group homomorphism ${\displaystyle Z:K({\mathcal {D}})\to \mathbb {C} }$, where ${\displaystyle K({\mathcal {D}})}$ is the Grothendieck group of ${\displaystyle {\mathcal {D}}}$, called a central charge, satisfying

• if ${\displaystyle 0\neq E\in {\mathcal {P}}(\varphi )}$ then ${\displaystyle Z(E)=m(E)\exp(i\pi \varphi )}$ for some strictly positive real number ${\displaystyle m(E)\in \mathbb {R} _{>0}}$.

It is convention to assume the category ${\displaystyle {\mathcal {D}}}$ is essentially small, so that the collection of all stability conditions on ${\displaystyle {\mathcal {D}}}$ forms a set ${\displaystyle \operatorname {Stab} ({\mathcal {D}})}$. In good circumstances, for example when ${\displaystyle {\mathcal {D}}={\mathcal {D}}^{b}\operatorname {Coh} (X)}$ is the derived category of coherent sheaves on a complex manifold ${\displaystyle X}$, this set actually has the structure of a complex manifold itself.

It is shown by Bridgeland that the data of a Bridgeland stability condition is equivalent to specifying a bounded t-structure ${\displaystyle {\mathcal {P}}(>0)}$ on the category ${\displaystyle {\mathcal {D}}}$ and a central charge ${\displaystyle Z:K({\mathcal {A}})\to \mathbb {C} }$ on the heart ${\displaystyle {\mathcal {A}}={\mathcal {P}}((0,1])}$ of this t-structure which satisfies the Harder–Narasimhan property above.[2] An element ${\displaystyle E\in {\mathcal {A}}}$ is semi-stable (resp. stable) with respect to the stability condition ${\displaystyle (Z,{\mathcal {P}})}$ if for every surjection ${\displaystyle E\to F}$ for ${\displaystyle F\in {\mathcal {A}}}$, we have ${\displaystyle \varphi (E)\leq ({\text{resp.}}<)\,\varphi (F)}$ where ${\displaystyle Z(E)=m(E)\exp(i\pi \varphi (E))}$ and similarly for ${\displaystyle F}$.