Can anyone explain why Mathematica is complaining here? It seems all the arguments of Prime[] in In[1] are obviously integers. It's not my fault if it uses floats. (And is there any way to coax it to get the exact answer, whose value I leave as a little puzzle?) ----- In[1]:= 1/2 + Sum[(1/Prime[n])Product[(1-1/Prime[k]),{k,1,n-1}],{n,2,Infinity}] 1 Product[1 - --------, {k, 1, -1 + n}] 1 Prime[k] Out[1]= - + Sum[-------------------------------------, {n, 2, Infinity}] 2 Prime[n] In[2]:= N[%] Prime::intpp: Positive integer argument expected in Prime[17.]. Prime::intpp: Positive integer argument expected in Prime[18.]. Prime::intpp: Positive integer argument expected in Prime[19.]. General::stop: Further output of Prime::intpp will be suppressed during this calculation. Out[2]= 0.897804 ----- --Dan