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Can the notion of vector space or algebra over a field be meaningfully extended to fractional dimensions, so that for example $\mathbb{R}^{-2/3}$ makes sense? Has this been explored somewhere?

I know that super vector spaces can be thought of as one way of generalizing vector spaces to negative integer dimensions. Is there a similar concept for dimensions that are rational numbers? I'm not talking about Hausdorff dimension, because it doesn't allow for negative rationals, and I'm rather looking for extensions from a more algebraic point of view (dimension as the trace of the identity map), without recurring to a given metric.

pregunton
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  • i've never heard of super vector spaces. – mercio Oct 31 '15 at 20:32
  • @mercio They are used in physics, in the context of supersymmetry. See here. – pregunton Oct 31 '15 at 21:13
  • and those things can have negative dimension ? do you have an example where assigning them negative dimension is useful ? – mercio Oct 31 '15 at 21:20
  • @mercio I learned about it from this answer. – pregunton Oct 31 '15 at 21:37
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    Super vector spaces are nothing but usual vector spaces that are $\Bbb Z _2$-graded. Anything mentioning "negative dimensions" is pure nonsense that you should ignore. – Alex M. Oct 31 '15 at 22:17
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    @AlexM.: if we define dimension as trace of the identity, it may be negative, since trace is trace of the even linear part minus trace of odd – ziggurism Nov 04 '15 at 20:08
  • @ziggurism: For endomorphisms of a superspace you have both a trace and a supertrace. If you use the second to define some sort of dimension, be honest and agree to call it "superdimension". In this case, I agree that a superspace may have a negative superdimension, but let us not confuse beginners by letting them mistakenly understand that the algebraic dimension may be negative. – Alex M. Nov 04 '15 at 22:07
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    @AlexM.: sure you may insist on pretending every word with "super-", but perhaps consider that not everyone does so, and the phrase "pure nonsense" may have been strong. – ziggurism Nov 04 '15 at 22:10
  • *"Prepending", that was meant to say – ziggurism Nov 05 '15 at 00:18
  • @ziggurism trace of the identity is always positive. You can define "supertrace" but it is another thing. – Anixx Jul 22 '24 at 00:18
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    @Anixx some people may prefer a single concept with a single word "trace". for them, then supertrace isn't another thing. it's the same thing, in another category. and then the trace of the identity may be negative. – ziggurism Jul 22 '24 at 14:31

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In fact, there are many examples where this happens. And as you suggested, it comes from an algebraic point of view; namely in the area of what are called fusion categories. These are categories which come with quite a bit of data to begin with. In particular, they are monoidal, there is some notion of simple objects, have duals and evaluation $$\epsilon:a\otimes a^*\longrightarrow 1$$ and coevaluation maps$$\hat{\epsilon}:\mathbb{1}\longrightarrow a\otimes a^*$$ associated to every object $a$. We can then define the trace of a morphism $f:a\longrightarrow a$ to be the composite $$1\xrightarrow{\epsilon}a\otimes a^* \xrightarrow{f\otimes id}a\otimes a^*\xrightarrow{\hat{\epsilon}}1$$ which is an element of the endomorphism ring of the unit object $1$ (this ring often happens to be a field). (For the sake of brevity I'll from here on assume that these categories are spherical, i.e. left trace agrees with right trace so we don't have to make any overly complicated distictions. If the category is not spherical we can still get some notion of dimension, called the squared norm of an object but I'm trying to keep this compact). The dimension of an object is then, as you suggested, defined to be $tr(id_a)$.

An example of such a category is the so-called Fibonnaci category. It has two simple objects, $X$ and $1$ with $X=X^* = $ $ ^{*}X$ and $X\otimes X= 1 \oplus X$. Using monoidality and additivity of all functors, we can then calculate $$dim(X) = (1+\sqrt{5})/2.$$ There is a plethora of such categories, this is just one example. I hope I could get the ideas across!

Patrick Schackmar
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