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Intuitively speaking, vector spaces are inherently endowed with a concept of “directionality”, since a vector is intuitively an arrow in some direction.

But if I’m not mistaken, we need to endow the vector space with an inner product to really talk about angles and directionality, but I’m not sure.

What is the most general structure on a set that formalizes the general intuitive notion of “direction”? (I am looking for something analogous to how topological spaces are the most general notion that formalizes “nearness” or “touchness”).

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  • $\begingroup$ a vector is just an element of a vector space (i.e. a structure with Abelian addition and compatible scalar multiplication with field elements), not "a direction". Direction for you means "coordinates" I think, so maybe $\mathbb{R}^I$ with some well-defined inner product (for angles) would be good enough. What do you need the generality for? $\endgroup$ – Henno Brandsma Feb 24 at 13:43
  • $\begingroup$ It's an interesting question to ask even if she doesn't "need" it for anything in particular. There are all sorts of questions one could ask. For example, can we definitely a meaningful notion of orthogonality even in spaces without a full inner product? $\endgroup$ – MJD Feb 24 at 16:15
  • $\begingroup$ The component functions of a Fourier decomposition are the basis of a vector space but the intuitive idea of "direction" is unclear in that space. The space does have an inner product and we might think of functions as "orthogonal" when their inner product is zero. I think you may have things backwards: it seems to me that "direction" requires more than an inner product, not less. $\endgroup$ – David K Feb 24 at 18:36
  • $\begingroup$ But in an inner product space the normalized inner product can be interpreted as the degree to which two vectors point in similar directions (with the similarity of orthogonal vectors equal to zero) and I think this is a very common point of view, even in function spaces. $\endgroup$ – MJD Feb 24 at 19:28
  • $\begingroup$ @MJD I agree, I would think of two functions as being "in a similar direction" or "almost orthogonal" by analogy with the way the inner product works on $\mathbb R^n,$ but unless I also imagine the functions as vectors in $\mathbb R^n$ I can't "see" the similarity of directions. So I think the idea of "direction" is not something that arises by natural intuition from the space itself; it's something we impose on it by analogy with something else. $\endgroup$ – David K Feb 24 at 21:21
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Every vector space has the concept "direction" in the form of equivalence classes of vectors: Two non-zero vectors point in the same direction if they are linearly dependent. The directions are then the equivalence classes of non-zero vectors. Let's call them “projective directions” because those equivalence classes are exactly how the projective space is constructed.

However with this definition, a vector points in the same direction as its negative, which is not quite what we commonly consider “the same direction”.

To go further, we need a vector space over an ordered field (such as the rational numbers, or the real numbers). Then we can refine that definition to say that two vectors point in the same direction if they are a positive multiple of each other. Again, the directions are the equivalence classes of vectors. Two of these exist for each projective direction, corresponding to a vector and its negative. We call the direction of $-v$ the opposite of the direction of $v$.

However our directions are still unrelated to each other, except for the concept of opposite direction. To go a bit further, we can assume a topological vector space, and look at the function that maps each vector to its direction. Then we can define a topology on the directions by just declaring that a set of directions is open iff the preimage of this set under the aforementioned function is open. This implies that the map of a vector to its direction is continuous. In particular, if a sequence of vectors converges to a specific vector, then the sequence of their directions converges to the limit vector's direction. This clearly is a desirable property.

I think this topological space of directions matches best the intuitive concept of “direction” when not introducing angles. In particular, we get the exact concepts of “same direction” and “opposite direction”, and “close directions” in the same sense as “close points” in a general topological space.

If you want to define the set of directions without referring to a vector space, those properties could be formalized as follows:

A direction space is a topological space together with a continuous bijection "opposite" with no fixed point, whose composition with itself gives the identity function.

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This isn't a very good answer, but so far it's the only one on offer. A matroid is a sort of general model of independence, which applies to a great many situations. One of these is that an arbitrary vector space can be construed as a matroid, with a set of vectors "independent" in the matroid sense if and only if they are linearly independent.

I think this is probably much more general than what you are hoping for, since its understanding of directionality is so coarse: it can only tell you when vectors point in the exact same direction. But maybe it will be of some use.

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