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Angle between two subspaces

`theta = subspace(A,B)`

`theta = subspace(A,B)` finds
the angle between two subspaces specified by the columns of `A` and `B`.
If `A` and `B` are column vectors
of unit length, this is the same as `acos(abs(A'*B))`.

Consider two subspaces of a Hadamard matrix, whose columns are orthogonal.

H = hadamard(8); A = H(:,2:4); B = H(:,5:8);

Note that matrices `A` and `B` are
different sizes — `A` has three columns and `B` four.
It is not necessary that two subspaces be the same size in order to
find the angle between them. Geometrically, this is the angle between
two hyperplanes embedded in a higher dimensional space.

theta = subspace(A,B) theta = 1.5708

That `A` and `B` are orthogonal
is shown by the fact that `theta` is equal to *π*/2.

theta - pi/2 ans = 0

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