Possible number of different neutrinos
Man'ko V. I., Markov M. A.
The number of different types of neutrinos may be related to the dimensionality of our space. This hypothesis is illustrated by an old model (dating from 1955) of a four-dimensional relativistic oscillator in which "spirits" are exorcised by an additional condition analogous to the Virasoro conditions (1970) for a string or, in another version, by an additional term in the equation itself.