Performances piecewise defined functions in analytic form,   prime-counting function, $\xi$ sets
  The article discusses the representation of discrete functions defined in an analytic form without the use of approximations, namely the Heaviside function, identity function, the Dirac delta function and the prime-counting function. Also in the article introduced and considered a new type of sets ($\xi$ set) by analogy taken from finding the sum of a number of grants and other contradictions in mathematics and physics. With the help of $\xi$ sets interpreted Russell's paradox in the system of axioms of naive set theory. 
Oleh Kyrhan
math.CA