Formal aspects of the Melosh and generalized unitary transformations of a class of massive-particle wave equations

The Melosh, Foldy--Wouthuysen, Cini--Touschek, and Majorana transformations of the Dirac Hemiltonian are discussed in a parallel way as special cases of two general unitary transformations for spin 1/2, which generalize to include all integer and half-integer spins for the Weaver--Hammer--Good class of massive-particle wave equations. It is further shown how to construct a matrix tilde α3 for all spins, the generalization of α3 for spin 1/2; and a discussion of ''good'' and ''bad'' components of operators and wave functions is given