ConceptComplete

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

Definition6.6Heart

The heart of a t-structure (T≀0,Tβ‰₯0)(\mathcal{T}^{\leq 0}, \mathcal{T}^{\geq 0}) on a triangulated category T\mathcal{T} is the full subcategory

A=T≀0∩Tβ‰₯0\mathcal{A} = \mathcal{T}^{\leq 0} \cap \mathcal{T}^{\geq 0}

The cohomological functor H0:Tβ†’AH^0 : \mathcal{T} \to \mathcal{A} is defined by H0(X)=τ≀0Ο„β‰₯0XH^0(X) = \tau_{\leq 0} \tau_{\geq 0} X.


Examples

ExampleStandard heart is A

For the standard t-structure on D(A)D(\mathcal{A}), the heart is D≀0∩Dβ‰₯0={Aβˆ™:Hn(A)=0Β forΒ nβ‰ 0}β‰…AD^{\leq 0} \cap D^{\geq 0} = \{A^\bullet : H^n(A) = 0 \text{ for } n \neq 0\} \cong \mathcal{A}. The identification sends a complex to its H0H^0.

ExamplePerverse sheaves

The heart of the perverse t-structure on Dcb(X)D^b_c(X) is the abelian category Perv(X)\mathrm{Perv}(X) 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.

ExampleTilted heart

For a torsion pair (T,F)(\mathcal{T}, \mathcal{F}) in A\mathcal{A}, the tilted heart in Db(A)D^b(\mathcal{A}) is the subcategory of complexes Aβˆ™A^\bullet with H0(A)∈FH^0(A) \in \mathcal{F}, Hβˆ’1(A)∈TH^{-1}(A) \in \mathcal{T}, and Hn(A)=0H^n(A) = 0 for nβ‰ 0,βˆ’1n \neq 0, -1. This new abelian category is called the tilt of A\mathcal{A} at (T,F)(\mathcal{T}, \mathcal{F}).

ExampleHeart of weight structure

For a weight structure on T\mathcal{T}, 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 T\mathcal{T} has a "weight filtration."

ExampleCategory O as a heart

The BGG category O\mathcal{O} for a semisimple Lie algebra can be realized as the heart of a certain t-structure on Db(O)D^b(\mathcal{O}) (the standard one). Non-standard t-structures give "exotic" versions of category O\mathcal{O}.

ExampleHeart of D^b(Coh(P^1))

On Db(Coh(P1))D^b(\mathrm{Coh}(\mathbb{P}^1)), the standard heart is Coh(P1)\mathrm{Coh}(\mathbb{P}^1). Tilting at the torsion pair (torsion sheaves, vector bundles) gives a new heart equivalent to mod-k[βˆ™β‡‰βˆ™]\mathrm{mod}\text{-}k[\bullet \rightrightarrows \bullet] (representations of the Kronecker quiver).

ExampleShort exact sequences in the heart

A short exact sequence 0→A→B→C→00 \to A \to B \to C \to 0 in the heart A\mathcal{A} corresponds to a distinguished triangle A→B→C→A[1]A \to B \to C \to A[1] in T\mathcal{T} with all three vertices in A\mathcal{A}. The connecting map C→A[1]C \to A[1] lies in ExtA1(C,A)\mathrm{Ext}^1_{\mathcal{A}}(C, A).

ExampleHeart and cohomology

The nn-th cohomology with respect to a t-structure is Hn=H0∘[βˆ’n]:Tβ†’AH^n = H^0 \circ [-n] : \mathcal{T} \to \mathcal{A}. This is a cohomological functor: it sends distinguished triangles to long exact sequences in A\mathcal{A}.

ExampleMultiple hearts on the same category

Db(Coh(P2))D^b(\mathrm{Coh}(\mathbb{P}^2)) admits infinitely many t-structures with different hearts. The space of Bridgeland stability conditions parametrizes these t-structures (and their central charges).

ExampleHeart determines the t-structure

The heart A\mathcal{A} determines the t-structure: T≀0={X:Hn(X)=0Β forΒ n>0}\mathcal{T}^{\leq 0} = \{X : H^n(X) = 0 \text{ for } n > 0\} and Tβ‰₯0={X:Hn(X)=0Β forΒ n<0}\mathcal{T}^{\geq 0} = \{X : H^n(X) = 0 \text{ for } n < 0\}. This follows from the axioms.

ExampleRealization functor

For a bounded t-structure with heart A\mathcal{A}, the realization functor real:Db(A)β†’T\mathrm{real} : D^b(\mathcal{A}) \to \mathcal{T} extends the inclusion Aβ†ͺT\mathcal{A} \hookrightarrow \mathcal{T}. It is always exact but not always an equivalence.

ExampleD-modules as a heart

On a smooth variety XX, the category of regular holonomic D-modules is the heart of a natural t-structure on Drhb(DX)D^b_{\mathrm{rh}}(D_X). The Riemann-Hilbert correspondence identifies this heart with perverse sheaves.


Theorem6.4Heart is abelian

The heart A=T≀0∩Tβ‰₯0\mathcal{A} = \mathcal{T}^{\leq 0} \cap \mathcal{T}^{\geq 0} of a t-structure on T\mathcal{T} is an abelian category. The kernel of f:Aβ†’Bf : A \to B in A\mathcal{A} is H0(Cone(f)[βˆ’1])H^0(\mathrm{Cone}(f)[-1]) and the cokernel is H0(Cone(f))H^0(\mathrm{Cone}(f)).

RemarkSignificance

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.