Null Space

A null space is a particular category of subspaces. The null space of a system of equations is the set of solutions for which the dependent variables cancel out.

Definition

In linear algebra, the null space is the subspace that satisfies Ax = 0. It is notated as N(A).

Null spaces are composed of null space vectors. There is always a zero vector in the null space. If a matrix is invertible however, the zero vector is the only null space vector.

Null space vectors have an identity property. Given any valid solution to Ax = b, any combination of null spaces can be added to that solution to create another valid solution, because b + 0 = b.

Solutions

Leaving aside the invertible case, the non-zero vectors of the null space can be solved for.

Consider the below system of equations.

w + 2x + 2y + 2z = 1
2w + 4x + 6y + 8z = 5
3w + 6x + 8y + 10z = 6

This system is formulated as a matrix and eliminated into:

┌           ┐ ┌  ┐   ┌  ┐
│[1] 2  2  2│ │ w│   │ 1│
│ 0  0 [2] 4│ │ x│ = │ 3│
│ 0  0  0  0│ │ y│   │ 0│
└           ┘ │ z│   └  ┘
              └  ┘

The number of vectors in the null space match the number of free variables. In this case, there are 2.

The null space vectors can be 'prepopulated' or 'templated' by allowing pivot variables to vary and alternating which free variable is set to 0 or 1. In this case, they are:

• [w 1 y 0]

• [w 0 y 1]

Solve Ax = 0 using these values. For example, substituting in the first vector's values gives...

w + 2x + 2y + 2z = 0
w + 2(1) + 2y + 2(0) = 0
w + 2 + 2y = 0
w = -2 - 2y

2w + 4x + 6y + 8z = 0
2w + 4(1) + 6y + 8(0) = 0
2w + 4 + 6y = 0

2w + 4 + 6y = 0
2(-2 - 2y) + 4 + 6y = 0
-4 - 4y + 4 + 6y = 0
2y = 0
y = 0

w = -2 - 2y
w = -2 - 2(0)
w = -2

...a null space vector in [-2 1 0 0].

The second null space vector is [2 0 -1 1].

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