How can I rationalize the denominator of an expression?

Question

Mathematica doesn't rationalize the denominator automatically, and I haven't found anything in the documentation about it. But I found an old post on MathGroup, which proposes a solution using ComplexityFunction.

In very simple cases, it works fine, but it fails in more complex cases. Is there a better way to do it?

Answer 1

RatDenom[x_]:=
  Module[{y,nn,dd,f,g,c,k,blah},
    (y=Together[x];
     nn=Numerator[y];
     dd=Denominator[y];
     f=MinimalPolynomial[dd,t];
     c=f /. t -> 0;
     g=Factor[(c-f)/t];
     {k, blah}=FactorTermsList[Expand[nn*(g /. t -> dd)]];
     Sign[c] ((k/GCD[k,c])*blah)/HoldForm[Evaluate@Abs[c/GCD[k,c]]])]

Write $x=\nu/\delta$. The point is that $f(\delta)=0$, so $g(\delta)=c/\delta$. Expand[]ing $g(\delta)$ produces an expression without denominators. We call Together[] before doing anything else so that RatDenom[1+1/(Sqrt[2]+1)] will work.

There are two formatting hacks, both of which were suggested by J.M. in comments on my deleted answer: The point of the FactorTermsList[] is to get RatDenom[1/(Sqrt[2]+Sqrt[5])] to output (-Sqrt[2]+Sqrt[5])/3, rather than (-3 Sqrt[2]+3 Sqrt[5])/9. The HoldForm[] is to get RatDenom[1/Sqrt[2]] to be Sqrt[2]/2, not 1/Sqrt[2].

The following output shows the strengths and limitations of this method:

(* A straight forward example *)

In[58]:= RatDenom[1/(Sqrt[2]+Sqrt[3]+Sqrt[5])]

         3 Sqrt[2] + 2 Sqrt[3] - Sqrt[30]
Out[58]= --------------------------------
                        12

(* Evaluate[] knows how to multiply expressions with Sqrt[11] *)

In[59]:= RatDenom[(3+Sqrt[11])/(4+Sqrt[11])]

         1 + Sqrt[11]
Out[59]= ------------
              5
(* Nested radicals are fine *)

In[60]:= RatDenom[(2+Sqrt[3])/(1+Sqrt[5+Sqrt[11]])]

Out[60]= (-8 - 4 Sqrt[3] + 2 Sqrt[11] + Sqrt[33] + 8 Sqrt[5 + Sqrt[11]] + 4 Sqrt[3 (5 + Sqrt[11])] - 2 Sqrt[11 (5 + Sqrt[11])] -  Sqrt[33 (5 + Sqrt[11])]) / 5

(* The outermost operation after Together[] must be division. *)

In[65]:= RatDenom[Sqrt[(1+Sqrt[2])/(1+Sqrt[3])]]

              1 + Sqrt[2]
         Sqrt[-----------]
              1 + Sqrt[3]
Out[65]= -----------------
                 1

(* Expand doesn't realize that this numerator equals 1 .*)

In[67]:= RatDenom[Sqrt[3+2 Sqrt[2]]/(1+Sqrt[2])]

           Sqrt[3 + 2 Sqrt[2]] - Sqrt[2 (3 + 2 Sqrt[2])]
Out[67]= -(---------------------------------------------)
                                 1

(* As we can confirm by using N[]. *)

In[68]:= N[%]

         1.
Out[68]= --
         1.

Answer 2

The following works on all the examples in David's answer. It uses the code provided by J.M. in the comments. The transformation is first tried on the whole expression, and if that fails it is applied separately to the numerator and denominator.

ratd[x_] := Module[{v},
  v = FullSimplify@ToRadicals@RootReduce@x;
  If[NumberQ[v], v, If[FreeQ[v, Root],
    With[{n = Numerator[v], d = Denominator[v]}, HoldForm[n/d]],
    With[{
      n = FullSimplify@ToRadicals@RootReduce@Numerator@x,
      d = FullSimplify@ToRadicals@RootReduce[1/Denominator@x]
      }, HoldForm[n d]]]]];

test = {1/(Sqrt[2] + Sqrt[3] + Sqrt[5]), (3 + Sqrt[11])/(4 + Sqrt[11]), 
  (2 + Sqrt[3])/(1 + Sqrt[5 + Sqrt[11]]), Sqrt[(1 + Sqrt[2])/(1 + Sqrt[3])], 
  Sqrt[3 + 2 Sqrt[2]]/(1 + Sqrt[2])};

ratd/@test