nLab dual morphism



Monoidal categories

monoidal categories

With braiding

With duals for objects

With duals for morphisms

With traces

Closed structure

Special sorts of products



Internal monoids



In higher category theory




The notion of dual morphism is the generalization to arbitrary monoidal categories of the notion of dual linear map in the category Vect of vector spaces.



Given a morphism f:Xβ†’Yf \colon X \to Y between two dualizable objects in a monoidal category (π’ž,βŠ—)(\mathcal{C}, \otimes), the corresponding dual morphism

f *:Y *β†’X * f^\ast \colon Y^\ast \to X^\ast

is the one obtained by ff by using the duality unit of XX (the coevaluation map) and the duality counit of YY (the evaluation map) as follows:

Y *β†’Y *βŠ—Iβ†’Y *βŠ—XβŠ—X *β†’Y *βŠ—YβŠ—X *β†’IβŠ—X *β†’X * Y^* \to Y^*\otimes I \to Y^*\otimes X\otimes X^* \to Y^*\otimes Y\otimes X^* \to I\otimes X^* \to X^*

This notion is a special case of the the notion of mate in a 2-category.

Namely if K≔B βŠ—π’žK \coloneqq \mathbf{B}_\otimes \mathcal{C} is the delooping 2-category of the monoidal category (π’ž,βŠ—)(\mathcal{C}, \otimes), then objects of π’ž\mathcal{C} correspond to morphisms of KK, dual objects correspond to adjunctions and morphisms in π’ž\mathcal{C} correspond to 2-morphisms in KK. Under this identification a morphism f:Xβ†’Yf \colon X \to Y in π’ž\mathcal{C} may be depicted as a 2-morphism of the form

* β†’πŸ™ * Y↓ ⇙ f ↓ X * β†’πŸ™ * \array{ \ast &\stackrel{\mathbb{1}}{\to}& \ast \\ {}^{\mathllap{Y}}\downarrow &\swArrow_{\mathrlap{f}}& \downarrow^{\mathrlap{X}} \\ \ast &\underset{\mathbb{1}}{\to}& \ast }

and duality on morphisms is then given by the mate bijection

* β†’πŸ™ * Y↓ ⇙ f ↓ X * β†’πŸ™ *↦* β†’πŸ™ * X *↓ ⇙ f * ↓ Y * * β†’πŸ™ *≔* β†’Y * * β†’πŸ™ * β†’πŸ™ * πŸ™β†“ ⇙ Ο΅ Y Y↓ ⇙ f ↓ X ⇙ Ξ· X ↓1 * β†’πŸ™ * β†’πŸ™ * β†’X * *. \array{ \ast & \overset{\mathbb{1}}{\to} & \ast \\ {}^{\mathllap{Y}} \downarrow & \swArrow_{\mathrlap{f}} & \downarrow^{\mathrlap{X}} \\ \ast & \underset{\mathbb{1}}{\to} & \ast } \;\;\;\;\; \mapsto \;\;\;\;\; \array{ \ast & \overset{\mathbb{1}}{\to} & \ast \\ {}^{\mathllap{X^\ast}} \downarrow & \swArrow_{\mathrlap{f^\ast}} & \downarrow^{\mathrlap{Y^\ast}} \\ \ast & \underset{\mathbb{1}}{\to} & \ast } \;\;\;\; \coloneqq \;\;\;\; \array{ \ast & \overset{Y^\ast}{\to} & \ast & \overset{\mathbb{1}}{\to} & \ast & \overset{\mathbb{1}}{\to} & \ast \\ {}^\mathllap{\mathbb{1}}\downarrow & \swArrow_{\epsilon_Y} & {}^{\mathllap{Y}} \downarrow & \swArrow_{f} & \downarrow^{\mathrlap{X}} & \swArrow_{\eta_X} & \downarrow \mathrlap{1} \\ \ast & \underset{\mathbb{1}}{\to} & \ast & \underset{\mathbb{1}}{\to} & \ast & \underset{X^\ast}{\to} & \ast } \,.



In π’ž=\mathcal{C} = Vect with its standard tensor product monoidal structure, a dual object is a dual vector space and a dual morphism is a dual linear map.


If AA, BB are C*-algebras which are Poincaré duality algebras, hence dualizable objects in the KK-theory-category, then for f:A→Bf \colon A \to B a morphism it is K-oriented, the corresponding Umkehr map is (postcomposition) with the dual morphism of its opposite algebra version:

f!≔(f op) *. f! \coloneqq (f^op)^\ast \,.

See at KK-theory – Push-forward in KK-theory.


More generally, twisted Umkehr maps in generalized cohomology theory are given by dual morphisms in (∞,1)-category of (∞,1)-modules. See at twisted Umkehr map for more.

Last revised on July 22, 2018 at 17:19:31. See the history of this page for a list of all contributions to it.