Abstract: 

A first-order countable language of signature. T(A) is the set of all sentences true in model A. This set of sentences is called an elementary type.Models of this elementary type and the similarity relation were considered. Two countable models A and B are similar if they are elementarily embeddable. Let BT(ω) be the number of classes of countable models with respect to the similarity relation. The known function I() and this function are distinct. Some properties of formula-definable semigroups of elementary types are also defined. Two types of model companions are considered.