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:
- a⊙(b⋅c)=a⊙b⋅a⊙c
- a⊙(b+c)=a⊙b+a⊙c
- a⊙b=b⊙a
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 a⊙b=0 for all a,b. I claim this is the only such operation. Indeed, we have a⊙c=a⊙(1⋅c)=(a⊙1)⋅(a⊙c).
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 a⋅a=a for all a), a⊙b=a⋅b is such an operation.
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