By the way, my
http://cplusplus.com/forum/lounge/58607/#msg316089 was a silly post. How did nobody notice?
Multiplication in R
2 is a surjection with or without 0. Also the definition of multiplication as a function R
2->R is silly, because then its inverse should be R->R
2, which division is not.
I'll try again. For all c in R/{0}, f(x) = c*x is a bijection. Then f
-1(x) = x/c. When c = 0, f(x) = 0. Still, in that case, by the definition that f
-1(x) = {y : f(y)=x}, f
-1(0) = R and f
-1(x) = {} when x!=0.
The thing to notice here is that division is not a group operation with or without 0. Really, a/b should be looked at as a*b
-1. My the definitions of this post, 0
-1 = 1/0 = {} and you can't multiply by an empty set.. (you could define that to be {} too; that would make sense)
@chrisname, no, those are from 16th century.