set and Sequence theory

https://en.wikipedia.org/wiki/Class_(set_theory)

https://en.wikipedia.org/wiki/Zermelo%E2%80%93Fraenkel_set_theory

https://books.google.it/books?id=dVncCl_EtUkC&pg=PA69

http://plato.stanford.edu/entries/set-theory/

http://www.rapidtables.com/math/symbols/Set_Symbols.htm

https://en.wikipedia.org/wiki/Set_theory

http://www.math.toronto.edu/weiss/set_theory.pdf

https://www.encyclopediaofmath.org/index.php/Axiomatic_set_theory

http://plato.stanford.edu/entries/set-theory/

http://people.umass.edu/gmhwww/595t/pdf/Set-Theory-Chap0.pdf

 http://math.stackexchange.com/questions/1353289/set-theory-formula

https://en.wikipedia.org/wiki/Set_theory

https://en.wikipedia.org/wiki/Sequence