Polynomial of finite degree actually infinite degree?

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Discussion Overview

The discussion revolves around the interpretation of the polynomial expression ##1+x+x^2## and its equivalence to the rational function ##\dfrac{1-x^3}{1-x}##. Participants explore the implications of defining polynomial degrees in the context of formal power series and the potential contradictions that arise when comparing finite and infinite degrees.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the left-hand side (LHS) has degree ##2## while the right-hand side (RHS) is expressed as an infinite series, suggesting a contradiction.
  • Others propose that the degree of a polynomial is defined as the highest degree of its monomials with non-zero coefficients, raising questions about how this definition applies in the context of infinite series.
  • A participant notes that expanding the product leads to cancellation of higher degree terms, implying no contradiction exists in that specific case.
  • Some participants argue that the LHS can be evaluated at ##x=1## while the RHS cannot, questioning the equality of the two expressions.
  • There is a discussion about the nature of formal power series versus polynomial functions, with some emphasizing that manipulations of formal series do not require convergence.
  • Concerns are raised about interpreting the notation of series and polynomials, with some participants suggesting that context is crucial for proper interpretation.
  • A later reply discusses the implications of treating polynomials over different fields, such as the boolean field GF(2), highlighting the diversity of polynomial functions and formal polynomials.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of polynomial degrees, the nature of formal power series, and the evaluation of the expressions involved. There is no consensus on how to resolve the apparent contradictions or on the proper interpretation of the expressions.

Contextual Notes

Participants note the importance of context in interpreting mathematical expressions, suggesting that without clear definitions or assumptions, conclusions drawn may be misleading. The discussion highlights the complexities involved in defining degrees and evaluating expressions in different mathematical frameworks.

Who May Find This Useful

This discussion may be of interest to those studying algebra, particularly in the areas of polynomial functions, formal power series, and the nuances of mathematical definitions and interpretations.

  • #31
Mr Davis 97 said:
##1+x+x^2 = \dfrac{1-x^3}{1-x} = (1-x^3)\cdot \dfrac{1}{1-x} = (1-x^3)\sum_{k=0}^\infty x^k##.

Isn't this a contradiction since the LHS has degree ##2## while the RHS has infinite degree?

By the usual definition of "polynomial" the RHS is not a polynomial. (The usual definition of polynomial doesn't permit a "polynomial" to be specified by the operation of taking a limit. Do your course materials actually define a type of "polynomial" that has "infinite degree"? )

A polynomial function f(x) may be equal to a function that is expressed in a way not permitted in the definition of "polynomial". For example f(x) = x = x + sin(x) - sin(x).

What qualifies a function to be a polynomial is that it may be expressed in a certain way. Alternative ways of expressing it don't involve any logical contradiction.
 

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