Transformations & Vector Spaces

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Understand the abstract structures that unify linear algebra.

Linear Transformations

A linear transformation T: V → W satisfies:

T(u + v) = T(u) + T(v)  (additivity)
T(cv) = cT(v)  (homogeneity)

Every linear transformation from ℝⁿ to ℝᵐ can be represented as multiplication by an m×n matrix. Geometric examples:

  • Rotation by θ: Uses sine and cosine in the matrix
  • Reflection: Across a line or plane
  • Scaling: Diagonal matrix with scale factors
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  • Projection: Onto a subspace — key in least-squares regression

These connect directly to coordinate geometry transformations.

Vector Spaces & Subspaces

A vector space V over ℝ is a set with addition and scalar multiplication satisfying 8 axioms (closure, associativity, commutativity, identity, inverse, compatibility, distributivity). Examples beyond ℝⁿ:

  • Polynomials of degree ≤ n — connects to polynomial algebra
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  • Continuous functions on [a, b] — connects to continuity in calculus
  • Solutions to homogeneous DEs — connects to differential equations

A subspace is a subset closed under addition and scalar multiplication. Important subspaces of a matrix A: column space (Col A), row space, null space (Nul A).

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Basis & Dimension

A basis is a linearly independent spanning set.
dim(V) = number of vectors in any basis
dim(ℝⁿ) = n, with standard basis e₁, e₂, …, eₙ

Change of basis transforms coordinates between different bases — essential when working with eigenvectors as a basis (diagonalization).

Rank & Nullity

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rank(A) = dim(Col A) = dim(Row A)
nullity(A) = dim(Nul A)
Rank-Nullity Theorem: rank(A) + nullity(A) = n
The Rank-Nullity Theorem is a dimension-counting result: the "input space" ℝⁿ splits into the part that maps to nonzero outputs (rank) and the part that maps to zero (nullity). This connects to solution counts for systems of equations: unique (full rank), infinite (nullity > 0), or none (inconsistent).
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