Fixed Points in Tense Models

We study into definability of least fixed points in tense logic. It is proved that least fixed points of tense positive $\Sigma$-operators are definable in transitive linear models. Examples are furnished showing that the least fixed points of tense positive operators may fail to be definable in the class of finite linearly ordered models, and the class of finite strictly linearly ordered models. Moreover, in dealing with the modal case, we point out examples of the non-definable inflationary points in the model classes mentioned.