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In Lurie's On the Classification of Topological Field Theories, he states in Proposition 1.1.8 that for an oriented compact manifold $M$ and a TQFT $Z:\mathrm{Cob}(n)\to \mathrm{Vect}_k$, there is a perfect pairing $Z(\overline{M})\otimes Z(M)\to k$ where $\overline{M}$ denotes $M$ with the opposite orientation. In the proof of this, he mentions the easily defined map $\alpha:Z(\overline{M})\to Z(M)^\vee$, where $^\vee$ denotes the dual space, which relies on the evaluation map coming from the cobordism $M\coprod\overline{M}\to\emptyset$ associated to $M\times[0,1]$. He intends to describe its inverse $\beta:Z(M)^\vee\to Z(M)^\vee\otimes Z(M)\otimes Z(\overline{M})\to Z(\overline{M})$ as a composite of the coevaluation map from $\emptyset\to M\coprod\overline{M}$ (i.e. $k\to Z(M)\otimes Z(\overline{M})$) followed by the bilinear pairing of $Z(M)$ with its dual. He then says that by "judiciously applying the axioms for a topological field theory, one can deduce that $\beta$ is an inverse to $\alpha$."

I am having trouble seeing that this is so. Just looking for maybe a little help, not necessarily the entire proof here, but anything would be appreciated? Is it futile to look at elements of the vector spaces considering that we don't really know much about what's going on there? Is this an entirely categorical proof, and if so, which "axioms" should I be looking the most closely at, the coherence ones for tensor functors?

Thanks!

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    Well, thinking about the triangle identities is what I needed to do. I figured it out. Will write it up on here tomorrow or something.2012-01-10

1 Answers 1

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Okay, so first a little notation. Let $V=Z(M)$, $V'=Z(\overline{M})$ and $V^\vee=Hom(V,k)$, i.e. the dual space of $V$. Now, we want to basically show an isomorphism between $V'$ and $V^\vee$.

Let's recall that we have the following maps in $\mathrm{Cob}(M)$:

$e:M\coprod\overline{M}\to\emptyset$

$c:\emptyset\to\overline{M}\coprod M$

$1_M:M\to M\coprod\overline{M}\coprod M\to M$ and

$1_\overline{M}:\overline{M}\to \overline{M}\coprod M\coprod \overline{M}\to \overline{M}$

where the latter two maps are compositions of $c$ and $e$ and are equal to the identity by the text Dylan referenced (i.e. sort of S shaped manifolds, or Zorro's lemma?).

From these morphisms we get linear maps in $\mathrm{Vect}_k$ (I will still just use $c$ and $e$ and the category will be clear from context):

$e:V\otimes V'\to k$

$c:k\to V\otimes V'$

$1_V:V\to V\otimes V'\otimes V\to V$

$1_{V'}:V'\to V'\otimes V\otimes V'\to V'$.

By $eval$ and $coev$ we denote the standard evaluation and coevaluation maps associated to $V$ and its dual. Recall that these maps also satisfy triangle identities similar to above.

Since $e$ defines a bilinear pairing on $V\otimes V'$, we can define $\alpha:V'\to V^\vee$ such that $v'\mapsto e(-,v')$.

Next we can define a map $\beta=(1_{V^\vee}\otimes c)\circ (eval\otimes 1_{V'}):V^\vee\to V^\vee\otimes V\otimes V'\to V'.$ We show that $\beta\circ\alpha=1_{V'}$ and $\alpha\circ\beta=1_{V^\vee}$. This follows from the following diagrams, which are easily seen to be commutative:

$V'\overset{\alpha}\longrightarrow V^\vee\overset{1_{V^\vee}\otimes c}\longrightarrow V^\vee\otimes V\otimes V'\overset{eval\otimes 1_{V'}}\longrightarrow V'$

$\downarrow ~~~~~~~~~~~~~~~~~~~~~~~~~~~~\uparrow_{\alpha\otimes 1_{V}\otimes 1_{V'}}~~~~~\uparrow$

$\searrow\overset{1_{V'}\otimes c}\longrightarrow\longrightarrow V'\otimes V\otimes V'\overset{e\otimes 1_{V'}}\longrightarrow\nearrow$

$V^\vee\overset{1_{V^\vee}\otimes c}\longrightarrow V^\vee\otimes V\otimes V'\overset{eval\otimes 1_{V'}}\longrightarrow V'\overset{\alpha}\longrightarrow V^\vee$

$\downarrow ~~~~~~~~~~~~~~~~~~~~~~\downarrow_{1_{V^\vee}\otimes 1_{V}\otimes\alpha}~~~~~~~~~~~~~~\uparrow$

$\searrow\overset{1_{V^\vee}\otimes coev}\longrightarrow V^\vee\otimes V\otimes V^\vee\overset{eval\otimes 1_{V^\vee}}\longrightarrow\nearrow$

Sorry for the horrific typesetting.

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    Hi @Bob , check out page 6 of the paper referenced by Dylan in the comments above for the triangle identities. Therein, they're given using string diagrams, but string diagrams can be decoded to give specific algebraic equalities. Looking now it looks like I wrote "triangle inequalities," which is obviously a mistake.2017-02-02