:The electromagnetic field is called a massless field because it has a certain natural correspondence with a theory of massless particles moving at the speed of light.
2.
Eilenberg's theorem, often referred to as the " variety theorem ", describes a natural correspondence between varieties of regular languages and pseudovarieties of finite semigroups.
3.
More generally, there is a natural correspondence between circles in the plane and points in three-dimensional projective space; a line in this space corresponds to a one-dimensional continuous family of circles called a pencil.
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