nLab delta-functor

Redirected from "∞-functors".
Note: n-functor, n-functor, and delta-functor all redirect for "∞-functors".

Contents

Definition

Let $\mathcal{A}$ , $\mathcal{B}$ be two abelian categories.

A homological $\delta$ -functor from $\mathcal{A}$ to $\mathcal{B}$ is for each $n \in \mathbb{N}$ a functor

T_n : \mathcal{A} \to \mathcal{B}

equipped for each short exact sequence $0 \to A \to B \to C \to 0$ in $\mathcal{A}$ with a natural transformation

\delta_n : T_n(C) \to T_{n-1}(A)

such that for each such short exact sequence there is, naturally a long exact sequence

\cdots T_{n+1}(C) \stackrel{\delta}{\to} T_n(A) \to T_n(B) \to T_n(C) \stackrel{\delta }{\to} T_{n-1}(A) \to \cdots \,.

The archetypical example is the chain homology functor

H_\bullet(-) : Ch_\bullet(\mathcal{A}) \to \mathcal{A}

from the category of chain complexes of some abelian category (for $\mathbb{N}$ -graded complexes).

The universal example are (non-total) right derived functors.

The notion is due to

A textbook account is for instance section 2.1 of

Last revised on September 30, 2017 at 00:00:16. See the history of this page for a list of all contributions to it.