Projections

When a vector does not exist in a column space, the projection is the best approximation of it in linear combinations of that column space.

Contents

  1. Projections
    1. Vectors
      1. Properties
    2. Matrices
      1. Properties
      2. Usage

Vectors

Given vectors a and b, b can be projected into C(a), the column space of a. This projection p has an error term e.

The factor which converts a into an estimate is notated as x̂, so that p = ax̂. The error term can be characterized by e = b - p or e = b - ax̂. a is orthogonal to e, so aT(b - ax̂) = 0. This simplifies to x̂ = (aTb)/(aTa). Altogether, the projection is p = a(aTb)/(aTa).

The projection matrix P satisfies p = Pb. It is calculated (aaT)/(aTa). C(P), the column space of P, is equivalent to the column space of a. (It follows that P is also of rank 1.)

Properties

The projection matrix P is symmetric (i.e. PT = P) and idempotent (i.e. P2 = P).

Matrices

Given a system as Ax = b, if b is not in C(A), the column space of A, then there is no possible solution for x. The best approximation is expressed as Ax̂ = p where projection p estimates b with an error term e.

The error term can be characterized by e = b - p or e = b - Ax̂. e is orthogonal to R(A), the row space of A; equivalently it is orthogonal to C(AT). Orthogonality in this context means that e is in the null space, so AT(b - Ax̂) = 0.

The system of normal equations is ATAx̂ = ATb. This simplifies to x̂ = (ATA)-1ATb. Altogether, the projection is characterized by p = A(ATA)-1ATb.

The projection matrix P satisfies p = Pb. It is calculated as P = A(ATA)-1AT.

b can also be projected onto e, which geometrically means projecting into the null space of AT. Algebraically, if one projection matrix has been computed as P, then the projection matrix for going the other way is (I - P)b.

Properties

As above, the projection matrix P is symmetric and idempotent.

If A is square, the above equations simplify rapidly.

If b actually was in C(A), then P = I. Conversely, if b is orthogonal to C(A), then Pb = 0 and b = e.

Usage

This should look familiar. A projection is inherently the minimization of the error term.

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