i'm pretty sure that it's fine if the rates are understood to be per year or some other finite period as is usually the case in economics after a year in the bank an amount of money m becomes m i where i is the nominal interest rate you can also write this as m r p where r is the real interest rate and p is the rate of inflation then you get the first equality for r it reduces to i p when p is small or if the rates are measured instantaneously something that probably only a mathematicians would do