Why Does Multiplication of Negative Numbers Yield Positive Results?

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Discussion Overview

The discussion revolves around the properties of multiplication in number systems, particularly focusing on why the multiplication of negative numbers results in positive numbers. Participants explore the implications of these properties within the framework of algebraic structures like rings and the nature of different mathematical systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the necessity of the property that -a x b = b x (-a) and suggests an alternative system where this does not hold, proposing that multiplication of two negative numbers could yield a negative result.
  • Another participant discusses the implications of having an additive identity (0) and a multiplicative identity (1), arguing that certain properties must hold for a system to be considered a ring, which would be violated if the proposed alternative were adopted.
  • A participant mentions that while some systems may not be rings, labeling them as number systems is problematic, referencing a user named "doron shadmi" as an example.
  • One participant points out that the commutative property does not hold for matrices, suggesting that the discussion may not apply to all mathematical structures.
  • Another participant asserts that matrices do not form a division algebra and questions their classification as a number system, while also stating that the proofs provided are valid within the ring of matrices.
  • There is a philosophical inquiry about whether our number system is indeed a ring or could be conceptualized differently, such as a one-dimensional arrow towards infinity.
  • A later reply confirms that the number system is a ring, clarifying that the mathematical term "ring" is distinct from its everyday usage.

Areas of Agreement / Disagreement

Participants express differing views on the nature of number systems and the properties of multiplication, with some supporting the established definitions of rings and others proposing alternative interpretations. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference specific mathematical properties and structures, but there are unresolved assumptions regarding the definitions and implications of these systems. The discussion includes a mix of algebraic reasoning and philosophical considerations about the nature of mathematical entities.

anuj
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Why we have a number system where

-a x b = b x (-a)

i.e. why we don't have a system where

-a x b not equal to b x (-a)

and

-a x (-b) = - a x b
a x b = a x b

so that the multiplication/ divison of two -ve numbers results in a -ve number and that of +ve numbers into a +ve number.

Any comments
 
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let's suppose this system has an additive identity 0, and a multiplicative identity 1, and. then

1*0 = 1*(0+0) => 1*0=0 (similarly 0*0=0, in fact 0*a=0 for all a)

then, 0= 1*(0) = 1*(1-1) = 1*1 + 1*(-1) if we're to have multiplication behaving reasonably (ie distributively) and thus 1*(-1) = -(1*1)

thus if you were to require certain things to fail your system cannot be a ring, which would be a shame, since it's rather nice that a number system is a ring.

there a lots of systems which aren't rings, however calling them a number system is not reasonable.
 
matt grime said:
there a lots of systems which aren't rings, however calling them a number system is not reasonable.
like the number systems of the user: "doron shadmi"?
 
bugger, i'd not spotted that. guess i'd got used to the lack of such posts. the 'any comments' should have given it away. (apologies if anuj is indeed not he, otherwise, lock, anyone?)
 
That would be true if a and b are integers, reals or else. But if a and b are matrices... that's not true AB is not BA. (maybe because its mixed tensor nature :? )
 
matrices fail to commute for geometric reasons. nothing to do with mixed tensors. however matrices do not even form a division algebra, so fail one of the criteria given. besides, are they a number system? also the proofs i provide are valid in the ring of matrices anyway. (what does commutativity have to do with anything?)
 
matt grime said:
thus if you were to require certain things to fail your system cannot be a ring, which would be a shame, since it's rather nice that a number system is a ring.

Are we sure that our number system is a ring and not an one dimensional arrow pointing towards +infinity (the imaginary numbers and its arrow not considered). The two ends i.e. -infinity and +infinity are open ends of the ring.
 
Are we sure that our number system is a ring

Yes.

(But the mathematical word "ring" has nothing to do with the english word "ring")
 

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