# at.algebraic topology – Measure the failure of colimit to commute with taking free loops (or Hochschild homology)?

For a space $$X$$, let $$mathcal{L}X = mathrm{Maps}(S^1, X)$$ be the free loop space.

Inclusion of constant loops gives a natural map $$X to mathcal{L}X$$. This is not an isomorphism unless $$X$$ is contractible, because it splits the fibration $$Omega X to mathcal{L} X to X$$.

I would like to know that it is not an isomorphism “because” loops do not commute with (homotopy) colimits.

That is: suppose $$X$$ is presented as a CW complex, i.e. as a colimit of contractible spaces: $$X = mathrm{Colim}, D_alpha$$. Then the natural map

$$X = mathrm{Colim} , D_alpha = mathrm{Colim} , mathcal{L} D_alpha to mathcal{L} , mathrm{Colim} , D_alpha = mathcal{L} X$$

is (homotopic to) the inclusion of constant loops.

It is possible to see that $$X mapsto mathcal{L} X$$ is not an isomorphism by computing some invariant which, in general, measures the failure of loops to commute with colimits?

Here I have in mind that if I wanted to know how a functor failed to be exact, I would study its derived functors.

When I search the internet for loop spaces and colimits, invariably I find myself reading about “calculus of functors”.

Is the calculus of functors going to help me here?

In fact for my purposes I am ultimately interested in the analogous question for the functor “Hochschild homology” from dg categories to chain complexes, so would be perfectly happy with an answer to the above question after taking chains.

(As for what these purposes are, it has to do with a problem about dynamics in contact manifolds. An explanation of how that is related is a bit afield and perhaps too long to include here, but you can see this short note.)

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