Unfinished machine arithmatic

*To*: mathgroup at smc.vnet.net*Subject*: [mg7508] Unfinished machine arithmatic*From*: Ersek_Ted%PAX1A at mr.nawcad.navy.mil*Date*: Sat, 7 Jun 1997 03:48:48 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Consider the following lines involving a mixture of machine numbers and exact numbers. In[1]:= 15.0/0.125 +Log[1-Erf[3]] Out[1]= 109.28 In[2]:= 15.0/0.125 +Log[1-Erf[7]] In[2]= 120. +Log[1-Erf[7]] The first line evaluates without any trouble, but the second line doesn't evaluate to a number even though it has a numerical value of 68.472 (found using arbitrary precision). Question: Was the appropriate value given for Out[2]. Lets look at the next line. In[3]:= 15.0/0.125+Log[1-Erf[7.0]] (* all machine numbers *) Out[3]= -Infinity The numerical value of Out[2] is 68.472 and thats a long way from (-Infinity). I suppose it WRI decided the following: - Arbitrary precision should not be used in the case of In[2] for the sake of efficiency. - The result of In[2] will not be (-Infinity), because it is far from correct. - The only thing left is to give an incomplete calculation for Out[2]. I think the implementation is very good, but I thought users should be aware that the arithmatic can end up unfinished. Ted Ersek