Why is the Multiplicative Identity Positive and Not Negative?

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Discussion Overview

The discussion centers around the properties of multiplication involving negative numbers, specifically questioning why the multiplicative identity is defined as positive rather than negative. Participants explore the implications of different definitions and the underlying logic of multiplication rules in mathematics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the reasoning behind the definitions of multiplication involving -1 and suggests an alternative set of definitions.
  • Another participant references axioms and theorems related to multiplication and negative numbers, proposing proofs to support the established definitions.
  • Some participants discuss the implications of defining -1*-1 as -1, noting that it would violate the distributive law.
  • A participant introduces a grammatical analogy to explain the logic of negative multiplication, suggesting a relationship between mathematics and language.
  • There is a discussion about the possibility of different multiplicative identities and how they would affect mathematical structures.
  • One participant speculates about the consequences of using -1 as the multiplicative identity, including transformations of functions and slopes in coordinate systems.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of multiplication involving negative numbers. There is no consensus on the necessity or correctness of the established definitions, and the discussion remains unresolved.

Contextual Notes

Some arguments rely on specific axioms and theorems that are not universally accepted or may depend on particular mathematical frameworks. The discussion includes speculative reasoning about the consequences of alternative definitions.

  • #31
yes, I understand that I'm not talking about ordinary multiplication on the reals as the rest of the world knows it.

hmmm... ok, I'm startin to get it... but! what if -1 is the multiplicative identity?

how far back do the rules go? does it stop at the multiplicative identity? or is there a reason that the MI must be positive 1?
 
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  • #32
There is a reason that 1 is the mult ident and that is becaue the operation is * the multiplcation operator as we know it where n*m means add m up n times (n, m are positive integers) and which is extend to the rest of the integers as we invented them.

The element that is the identity with respect to some opereation is dependent on the operation.

can we talk about addition since that is simpler?

Take Z the integers with the usual operations of addition denoted as + , then defnie a new opertaion & where

x&y=x+y-1

then -1 is the identity with respect to this "addition". See, it can be done, but you are attempting to think of our declaration of identities (an inverses) as independent of an operation.
 
  • #33
Jonny_trigonometry said:
yes, I understand that I'm not talking about ordinary multiplication on the reals as the rest of the world knows it.

hmmm... ok, I'm startin to get it... but! what if -1 is the multiplicative identity?

how far back do the rules go? does it stop at the multiplicative identity? or is there a reason that the MI must be positive 1?
The multiplicative identity is defined such that if we let e be the multiplicative identity:

e*x = x = x*e

Just as the additive identity, say f, is defined as:

f+x = x = x+f
 

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