The principle of certainty of taxation is the dimension of a general requirement of certainty in the legal system. The purpose of this article is to argue the thesis that uncertainty in tax law is not always an absolute evil, sometimes it acts as a means of the most optimal (and in some cases the only possible) settlement of relations in the field of taxes. On the contrary, uncertainty and fragmentation in tax law are colossal problems subject to overcome by the efforts of scientists, legislators, judges, and practicing lawyers. Uncertainty in tax law is manifested in two ways: on the one hand, negatively—as a defect (omission) of the legislator and, on the other hand, positively—as a set of specific legal means and technologies that are purposefully used in lawmaking and law enforcement. In this context, relatively determined legal tools are an effective channel for transition from uncertainty to certainty in the field of taxation. A tendency towards increased use of relatively determined legal tools in lawmaking processes (for example, principles, evaluative concepts, judicial doctrines, standards of good faith and reasonableness, discretion, open-ended lists, recommendations, framework laws, silence of the law, presumptive taxation, analogy, etc.), and involving various actors (courts, law enforcement agencies and officials, international organizations, citizens, organizations and their associations) allow making tax laws more dynamic flexible, and adequate to changing realities of everyday life.

An inductive probabilistic approach to formal concept analysis (FCA) is proposed in which probability on formal contexts is considered; probabilistic formal concepts that have predictive force are defined: nonclassified objects can be assigned to earlier found probabilistic formal concepts; random attributes are eliminated from probabilistic formal concepts; probabilistic formal concepts are robust with respect to data noise. A result of experiment is presented in which formal concepts (in their standard definition in FCA) are first distorted by random noise and then recovered by detecting probabilistic formal concepts.

The subject.The author studies models of tax administration.The purpose of the articleis to confirm or refute hypothesis that successful tax administration strategy consists a balanced combination of tax enforcement measures and positive incentives.The methodologyof the study includes content analysis of the scientific papers prepared by European and American lawyers and economists.The main results, scope of application. Tax compliance is an essential component of a wellfunctioning tax system. The forms and methods by which states ensure tax compliance, and taxpayers try to avoid paying taxes, change significantly over time, due to the modernization of tax policy, changes in the alignment and transformation of economic forces, institutions, norms and technologies, Historically, tax relations have always been strictly vertical and hierarchical, relying on administrative-command methods of tax administration, and also characterized by retrospective tax audits and the threat of imposing harsh sanctions to ensure tax compliance. The one-sided nature and lack of flexibility of such a model, limited by the triad of regulatory instruments “obligation – control – responsibility”, was initially subjected to comprehensive criticism in the scientific community.Revolutionary transformations in public administration practice have an impact on all aspects of tax interactions, including tax compliance and tax administration. The global trend lies in the understanding that tax administration, in essence, should be expressed not in the confrontation between taxpayers and tax authorities, but in their dialogue, interaction and cooperation.The flexible tax administration model is based on new approaches to public administration based on a shift in emphasis from coercion to persuasion, from rigidity to flexibility, from “reactivity” to “proactivity”, from “influence” to “interaction” between managers and those controlled. Today, fiscal authorities are encouraged to adhere to a «cooperative approach» in dealing with taxpayers, based on principles such as dialogue, mutual understanding, impartiality, reciprocity, transparency, compromise and flexible response. At the same time, law-abiding behavior should be encouraged in every possible way, and unlawful behavior should be subject to increased discouragement.The doctrine of flexible tax administration advocates the development of trust in the relationship between taxpayers and tax authorities, offering the latter a wide range of tools to ensure tax compliance, and not limited to control and sanctions. The main mission of the state is to encourage taxpayers to partner with fiscal authorities and to voluntary compliance.For any state, the most desirable situation is the voluntary fulfillment of tax obligations by all participants in tax interactions, which makes it possible to minimize the size of the tax gap. Both persuasion and coercion as methods of tax administration have their pros and cons. Therefore, the key to a successful tax administration strategy is not abandoning one method in favor of another, but combining them in a balanced manner so that both methods cumulatively complement each other.Conclusions.In order to effectively increase the level of tax compliance, it is necessary to use a comprehensive tax-legal, organizational and informational toolkit, including both tax enforcement measures demonstrating the tax administration's determination to ensure tax compliance on the part of tax violators, and measures of positive incentives and rewards oriented for service support of law-abiding and conscientious taxpayers.