Re: Bug 1+4/10

*To*: mathgroup at smc.vnet.net*Subject*: [mg120107] Re: Bug 1+4/10*From*: Bill Rowe <readnews at sbcglobal.net>*Date*: Fri, 8 Jul 2011 04:55:01 -0400 (EDT)

On 7/7/11 at 7:32 AM, slawek at host.pl (slawek) wrote: >If Mathematica answer that a == b is true, then Mathematica should >also answer that N[a] == N[b] is true. It is trivial. Agreed. And Mathematica does this using your example, i.e. In[2]:= N[1.4] == N[1 + 4/10] Out[2]= True >You can, poor boy, check that 1.4 == 1+1/4 gives True, but N[1.4, >30] gives different result than N[1+1/4, 30] . You need to understand/learn the difference between N[1.4] an identity operation, i.e. In[4]:= 1.4 === N[1.4] Out[4]= True and N[1.4, 30] another identity operation, i.e. In[5]:= 1.4 === N[1.4, 30] Out[5]= True In[6]:= Precision[N[1.4, 30]] Out[6]= MachinePrecision and N[1+4/10] an operation that decreases information There is a very large set of values for x such that N[x] will be 1.4. Specifically, N[1.4 + a $MachineEpsilong]==N[1.4] will return true for any a such that Abs[a]<1 Since, all of these are equally valid choices, it would be incorrect for Mathemtica to select one over all others. Undoubtedly, this is why N[1.4, 30] is an operation that does nothing. In short, N is not intended to increase the precision of a number. There is no universally valid way to increase numerical precision. Any attempt to do so requires creation of new information which can be done in a large variety of ways.