Rotational invariance
In mathematics, a function defined on an inner product space is said to have rotational invariance if its value does not change when arbitrary rotations are applied to its argument.
For example, the function
is invariant under rotations of the plane around the origin, because for a rotated set of coordinates through any angle θ
the function takes exactly the same form
The rotation of coordinates can be expressed using matrix form using the rotation matrix,
or symbolically x′ = Rx. Symbolically, the rotation invariance of a real-valued function of two real variables is
In words, the function of the rotated coordinates takes exactly the same form as it did with the initial coordinates, the only difference is the rotated coordinates replace the initial ones. For a real-valued function of three or more real variables, this expression extends easily using appropriate rotation matrices.
The concept also extends to a vector-valued function f of one or more variables;
In all the above cases, the arguments (here called "coordinates" for concreteness) are rotated, not the function itself.
For a function
which maps elements from a subset X of the real line ℝ to itself, rotational invariance may also mean that the function commutes with rotations of elements in X. This also applies for an operator that acts on such functions. An example is the two-dimensional Laplace operator
which acts on a function f to obtain another function ∇2f. This operator is invariant under rotations.
If g is the function g(p) = f(R(p)), where R is any rotation, then (∇2g)(p) = (∇2f)(R(p)); that is, rotating a function merely rotates its Laplacian.
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