Heart of a t-structure
The heart of a t-structure is the abelian category sitting inside a triangulated category. It consists of objects that are simultaneously "non-positive" and "non-negative" with respect to the t-structure. The heart is the place where exact sequences live, even though the ambient triangulated category only has distinguished triangles.
Definition
The heart of a t-structure on a triangulated category is the full subcategory
The cohomological functor is defined by .
Examples
For the standard t-structure on , the heart is . The identification sends a complex to its .
The heart of the perverse t-structure on is the abelian category of perverse sheaves. Perverse sheaves are neither sheaves nor perverse; they are complexes of sheaves satisfying support and cosupport conditions with respect to a stratification.
For a torsion pair in , the tilted heart in is the subcategory of complexes with , , and for . This new abelian category is called the tilt of at .
For a weight structure on , the heart consists of objects that are both "non-negative" and "non-positive" in the weight sense. Unlike t-structure hearts, weight structure hearts are not necessarily abelian but are additive and have the property that every object of has a "weight filtration."
The BGG category for a semisimple Lie algebra can be realized as the heart of a certain t-structure on (the standard one). Non-standard t-structures give "exotic" versions of category .
On , the standard heart is . Tilting at the torsion pair (torsion sheaves, vector bundles) gives a new heart equivalent to (representations of the Kronecker quiver).
A short exact sequence in the heart corresponds to a distinguished triangle in with all three vertices in . The connecting map lies in .
The -th cohomology with respect to a t-structure is . This is a cohomological functor: it sends distinguished triangles to long exact sequences in .
admits infinitely many t-structures with different hearts. The space of Bridgeland stability conditions parametrizes these t-structures (and their central charges).
The heart determines the t-structure: and . This follows from the axioms.
For a bounded t-structure with heart , the realization functor extends the inclusion . It is always exact but not always an equivalence.
On a smooth variety , the category of regular holonomic D-modules is the heart of a natural t-structure on . The Riemann-Hilbert correspondence identifies this heart with perverse sheaves.
The heart of a t-structure on is an abelian category. The kernel of in is and the cokernel is .
The fact that the heart is abelian is remarkable: triangulated categories are not abelian (they lack kernels/cokernels), yet they contain abelian categories as hearts. This is how abelian categories arise "in nature" from triangulated categories, and it is the mechanism behind the construction of perverse sheaves, tilted algebras, and Bridgeland stability conditions.