Caldwell, William (2009), "Applying Concept Mapping to Algebra I", in Afamasaga-Fuata'i, Karoline (ed.), Concept Mapping in Mathematics: Research into Practice, Springer, pp. 217–234, doi:10.1007/978-0-387-89194-1_11, ISBN9780387891941; see section "Degree of a polynomial", pp. 225–226: "The product of the zero polynomial [with] any other polynomial is always the zero polynomial, so such a property of degrees (the degree of the product is the sum of the degrees of the two factors) would not hold if one of the two polynomials were the polynomial 0. That is why we do not assign a degree to the zero polynomial."
