Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Number system: a basic question

  1. Oct 19, 2004 #1
    Why we have a number system where

    -a x b = b x (-a)

    i.e. why we dont have a system where

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


    -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 in to a +ve number.

    Any comments
  2. jcsd
  3. Oct 19, 2004 #2

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    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.
  4. Oct 19, 2004 #3
    like the number systems of the user: "doron shadmi"?
  5. Oct 19, 2004 #4

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    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?)
  6. Oct 19, 2004 #5
    That would be true if a and b are integers, reals or else. But if a and b are matrices... thats not true AB is not BA. (maybe because its mixed tensor nature :? )
  7. Oct 19, 2004 #6

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    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?)
  8. Oct 20, 2004 #7
    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.
  9. Oct 20, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    (But the mathematical word "ring" has nothing to do with the english word "ring")
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Number system: a basic question
  1. Basics of numbers! (Replies: 29)