From: Juergen Koslowski <koslowj@iti.cs.tu-bs.de>
To: categories@mta.ca
Subject: more dagger problems
Date: Wed, 7 Mar 2007 02:21:22 +0100 (CET) [thread overview]
Message-ID: <E1HOkzh-0001Yi-VR@mailserv.mta.ca> (raw)
Besides their funny name, dagger-compact categories present, or
rather expose, another terminological dilemma: what's an adjoint?
Although the notion of dagger-compact category (dcc) was originally
defined for symmetric monoidal categories, let's try to makes sense of
it without symmetry. In fact, this should work in a (linear)
bicategory or even a poly-bicategory.
- The dagger operation flips 2-cells vertically (in view of the
picture calculus). f^{\dagger} is called the
``adjoint'' of f, which matches the terminology of functional
analysis and physics. In case of a linear bicategory, the two 1-cell
compositions \tensor for the domain 1-cells and \par for the
codomain 1-cells get interchanged as well.
- The definition of a dcc also calls for ``duals'' A^* of 1-cells,
which in graphical terms flips 1-cells horizontally.
- Finally, there are``units'' (or ``Bell states'' in physics terms)
\eta_A: I_Y ==> A^*\tensor A, when A:X --> Y.
The axioms for a symmetric dcc turn the ``duals'' turn into
categorical adjoints with unit \eta_A and counit (\eta_A)^{\dagger}
(the ``adjoint'' of the unit). This seems to require symmetry, but it
really does not. The correct interpretation of A^* should be that of
a 2-sided (categorical) adjoint for A (linear adjoint in the case of
poly-bicategories), i.e., A^* -| A -| A^*. Hence there are 2
categorical adjunctions and hence 2 units, besides \eta_A also
\eta_A^*: I_X ==> A\tensor A^*. Without symmetry (\eta_A)^{\dagger}
cannot be the counit for the adjunction A^* -| A, but for the other
adjunction A -| A^*. Unfortunately, the functional analysis
terminology would refer to the counit of the second adjunction as the
adjoint of the first adjunction's unit, which I find rather confusing.
This bicategorical view also clarifies that the star operation is an
involution on 1-cells, while dagger is an involution on 2-cells.
While the name ``{1,2}-involutive bicategory'' may be adequate,
``{1,2}-involutive monoidal category'' is quite a mouthful.
I seem to recall reading not too long ago that Kahn did _not_ pursue
an (often rumoured) analogy with functional analysis when introducing
(categorical) adjunctions, and here we see the actual mismatch.
-- Jürgen
--
Juergen Koslowski If I don't see you no more on this world
ITI, TU Braunschweig I'll meet you on the next one
koslowj@iti.cs.tu-bs.de and don't be late!
http://www.iti.cs.tu-bs.de/~koslowj Jimi Hendrix (Voodoo Child, SR)
next reply other threads:[~2007-03-07 1:21 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-03-07 1:21 Juergen Koslowski [this message]
2007-03-07 4:44 John Baez
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