Should that have been “between one and infinity”? Otherwise you cannot claim that 1/R is between zero and one.
I would prefer, given the domain, to amend to "given an R between 0 and infinity, I will return R for all R < 1, or 1/R otherwise". But yes, the proof was flawed.
Should that have been “between one and infinity”? Otherwise you cannot claim that 1/R is between zero and one.