Modifications of forms of relativistic dynamics related to the Pomcare transformations

Modifications of the forms of dynamics are defined algebraically; the general form of transformations between modifications is found; and all modifications of the point, instant, and front forms of dynamics and their limiting cases are described. It is shown that the limit of the modifications of the instant form – if the limit exists and is nontrivial – is in the general case a quasifront form of dynamics with a $5$-dimensional free subalgebra; in the absence of bound states and for the Bakamjian–Thomas model the limit is the front form of dynamics.