Linear Algebra
2.58 min read

Algebraic Properties of Vectors

Vector addition and scalar multiplication satisfy a rich set of algebraic properties. These properties hold for any vectors in Rm\mathbb{R}^m and any scalars, and they follow directly from the fact that real number arithmetic satisfies these rules entry-by-entry.

The most important properties are: commutativity (u+v=v+u\mathbf{u} + \mathbf{v} = \mathbf{v} + \mathbf{u}), associativity ((u+v)+w=u+(v+w)(\mathbf{u} + \mathbf{v}) + \mathbf{w} = \mathbf{u} + (\mathbf{v} + \mathbf{w})), and distributivity (c(u+v)=cu+cvc(\mathbf{u} + \mathbf{v}) = c\mathbf{u} + c\mathbf{v}).

These 8 properties — when abstracted from Rm\mathbb{R}^m — define what a vector space is. Any system satisfying these axioms behaves like vectors, even if its elements look nothing like number tuples (e.g., polynomials, matrices, continuous functions).

Formal View

Theorem 2.5 — Vector Space Properties
For all u,v,wRm\mathbf{u}, \mathbf{v}, \mathbf{w} \in \mathbb{R}^m and c,dRc, d \in \mathbb{R}: \begin{enumerate} \item u+v=v+u\mathbf{u} + \mathbf{v} = \mathbf{v} + \mathbf{u} (commutativity) \item (u+v)+w=u+(v+w)(\mathbf{u} + \mathbf{v}) + \mathbf{w} = \mathbf{u} + (\mathbf{v} + \mathbf{w}) (associativity) \item u+0=u\mathbf{u} + \mathbf{0} = \mathbf{u} (zero identity) \item u+(u)=0\mathbf{u} + (-\mathbf{u}) = \mathbf{0} (additive inverse) \item c(u+v)=cu+cvc(\mathbf{u} + \mathbf{v}) = c\mathbf{u} + c\mathbf{v} (distributivity over vector addition) \item (c+d)u=cu+du(c + d)\mathbf{u} = c\mathbf{u} + d\mathbf{u} (distributivity over scalar addition) \item c(du)=(cd)uc(d\mathbf{u}) = (cd)\mathbf{u} (scalar associativity) \item 1u=u1 \cdot \mathbf{u} = \mathbf{u} (scalar identity) \end{enumerate}

Why This Matters

These 8 axioms are the minimal requirements for a structure to behave like "vectors." They define the theory of vector spaces.

  • Abstract vector spaces include polynomials, matrices, and function spaces — all obeying the same algebra
  • These properties justify why linear algebra techniques transfer across domains
  • In machine learning, the 8 axioms ensure that gradient updates "add up" correctly

Quiz

Question 1

For any vector v\mathbf{v}, we have 0v=00 \cdot \mathbf{v} = \mathbf{0}.

Common Mistakes

  • Thinking commutativity holds for matrix multiplication — it does for vectors but NOT for matrices.
  • Confusing the zero vector 0\mathbf{0} with the scalar 00 — they are different objects.