… the formal definition of a vector

definitionvector-spacesvectors

This seems like a crazy question, I know. But my current math course in university has destroyed my understanding of what a vector is altogether:
We've learned that a vector space is a collection of vectors that satisfy the 3 properties (a vector space must be associated with a set of scalars called its Field, its elements must have some well-defined addition operation, and there must be an operation called scalar multiplication, etc.) However, when we got to things like function spaces and the fact that functions themselves can be thought of as vectors with "direction" (which lets us say things like $e^{i\theta}$ and $e^{-i\theta}$ are linearly independent), I started to lose all sanity. At this point, we're simply defining a vector as an element of a vector space, but to me, this gives way to circular reasoning: a vector is defined by its relation to a vector space and a vector space is defined by the behaviour of its associated vectors.

So I ask…what is a vector anymore? What do the formal definitions say?

Best Answer

A vector space is any space where you can do addition and scaling in a way that "makes sense". It does not nessesarily have to align with your intuition of containing "arrows" that point somewhere in physical space, that is just starting intuition for it.

The formal definition of a (real) vector space is that it's a set $V$, containing a special element $0$, whose elements can be added together and scaled by real numbers. It also has to follow the basic algebraic rules you'd expect it to follow. Formally, we have for all $r_1,r_2 \in \mathbb{R}$ and $v_1,v_2,v_3 \in V$,

$$(v_1 + v_2) + v_3 = v_1 + (v_2 + v_3),$$ $$0 + v_1 = v_1,$$ $$v_1 + (-1)\cdot v_1 = 0,$$ $$v_1 + v_2 = v_2 + v_1$$

And

$$r_1(r_2\cdot v_1) = (r_1r_2)\cdot v_1,$$ $$(r_1 + r_2)\cdot v_1 = r_1\cdot v_1 + r_2 \cdot v_1,$$ $$r_1\cdot (v_1 + v_2) = r_1\cdot v_1 + r_1 \cdot v_2.$$

Any set equipped with these operations that follows these rules is a vector space, and its elements are definitionally vectors.

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