“write a bivariate linear equation with the following property, or explain why this cannot be done. -> regardless of what value “x” has, the corresponding “y” -value is always positive.

the construction of fields of formal infinite series in several variables, generalizing the classical notion of formal Laurent series in one variable. Our discussion addresses the field operations for these series (addition, multiplication, and division), the composition, and includes an implicit function theorem.

(PDF) Formal Laurent series in several variables. Available from: https://www.researchgate.net/publication/259130653_Formal_Laurent_series_in_several_variables [accessed Oct 08 2018].