Saturday, 24 May 2014

abstract algebra - Commutative binary operations on BbbC that distribute over both multiplication and addition



Does there exist a non-trivial commutative binary operation on C that distributes over both multiplication and addition?



In other words, if our operation is denoted by , then I want the following to hold:




  1. a(bc)=abac

  2. a(b+c)=ab+ac

  3. ab=ba




All of the things I can find so far distribute over either multiplication or addition, but not both. Alternatively, is there a proof that no such operation can exist?



I wasn't sure if this question was too elementary for MO, so I'm trying here first. This question is obliquely related to the following other questions I've asked on MO and here:




Answer



One such operation is ab=0 for all a,b. I claim this is the only such operation. Indeed, we have ac=a(1c)=(a1)(ac).

Taking c=1 gives that a1 must be either 0 or 1 for each a. But if a1=1, then (a+a)1=2, which is impossible. So in fact a1=0 for all a, and now the equation above tells us ac=0 for all c as well.




This argument uses only the fact that distributes over multiplication on the left and distributes over addition on the right. With slight modification, it applies equally well with C replaced by any ring in which 2 is not a zero divisor. Note that in arbitrary rings, there can be other such operations . For instance, in a Boolean ring (in which aa=a for all a), ab=ab is such an operation.


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