rigid
Suppose is a collection of mathematical objects(for instance, sets, or functions).Then we say that is rigid if every is uniquely determined by less information about thanone would expect.
It should be emphasized that the above “definition” does notdefine a mathematical object. Instead, it describes in what sensethe adjective rigid is typically used in mathematics,by mathematicians.
Let us illustrate this by some examples:
- 1.
Harmonic functions on the unit disk are rigid in the sense thatthey are uniquely determined by their boundary values.
- 2.
By the fundamental theorem of algebra
, polynomials
in are rigid in the sense that any polynomial is completely determined byits values on any countably infinite
set, say , or the unit disk.
- 3.
Linear maps between vector spaces are rigid in the sense that any is completelydetermined by its values on any set of basis vectors of .
- 4.
Mostow’s rigidity theorem