CS223 · Annotated SlidesChapter 2 — Subspaces of ℝⁿ
Chapter 2 · Annotated slides

Subspaces of ℝⁿ

Subspaces, the column space and null space of a matrix, bases, finding bases for Col A (pivot columns of A) and Nul A (parametric solution of Ax = 0), coordinates, dimension and rank, and the Rank Theorem.

Slide text Study notes Highlight: key words Tips, tricks & traps Seen again elsewhere How it can come up
Slide 1: Subspaces of R
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  1. A subspace H of ℝⁿ contains 0 and is closed under addition and scalar multiplication.
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MCQ

Which is a subspace of ℝ²?

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    Slide 3: Example A line through the origin is another example of a subspace
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          Slide 6: In other words The column space is the same as the span of the columns of the matrix
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          1. Col A: span of A’s columns (subspace of ℝᵐ). Nul A: all solutions of Ax = 0 (subspace of ℝⁿ).
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              Slide 9: Example
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                        Slide 15: To find the basis of a To find the basis of the null column space of matrix A : space of matrix A, 1. Find the equivalent 1. Solve the homogenous matrix in Echelon Form equation A x = 0 or Reduced Echelon 2. Write the solution in
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                        1. Basis of Col A: the pivot columns of the ORIGINAL A (not of its echelon form). Basis of Nul A: the vectors in the parametric solution of Ax = 0.
                        Exam trapTake the pivot columns from A itself, not from the reduced matrix.
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                              1. Rank Theorem: rank A + dim Nul A = n (number of columns).
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                              MCQ

                              A 5×7 matrix has rank 4. dim Nul A =

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