Can I simplify an expression into form which uses my own definitions?

Question

This seems like a simple thing to do, but I couldn't find anything relevant from Mathematica documentation.

So suppose I have an expression:

a*b/(a + a*Cos[a/b])

And I have defined:

k=a/b

Now I want to simplify the expression above so that the simplify would use my definition of k in place of a/b in as many places as possible so that the final expression would look something like:

a/(k+k*Cos[k])

This was just a simple example I made up to demonstrate what I'd like to do, but I have encountered a similar situations many times every now and then.

Answer 1

Daniel Lichtblau and Andrzej Koslowski posted a solution in mathgroup, which I adjusted marginally. (I like to use german identifiers, because they will never clash with Mma builtins). That's the code:

SetAttributes[termErsetzung,Listable];
termErsetzung[expr_, rep_, vars_] := 
Module[{num = Numerator[expr], den = Denominator[expr],
        hed = Head[expr], base, expon},
  If[PolynomialQ[num, vars] && PolynomialQ[den, vars] && ! NumberQ[den], 
    termErsetzung[num, rep, vars]/termErsetzung[den, rep, vars], (*else*)
    If[hed === Power && Length[expr] === 2,        
       base  = termErsetzung[expr[[1]], rep, vars];
       expon = termErsetzung[expr[[2]], rep, vars];
       PolynomialReduce[base^expon, rep, vars][[2]],        (*else*)
      If[Head[Evaluate[hed]] === Symbol && 
        MemberQ[Attributes[Evaluate[hed]], NumericFunction], 
        Map[termErsetzung[#, rep, vars] &, expr],    (*else*)
       PolynomialReduce[expr, rep, vars][[2]] ]]]
];

TermErsetzung[rep_Equal,vars_][expr_]:=
  termErsetzung[expr,Evaluate[Subtract@@rep],vars]//Union;

Usage is like this:

a*b/(a + a*Cos[a/b]) // TermErsetzung[k b == a, b]

a/(k (1 + Cos[k]))

The first parameter is the "replacement equation", the second the variable (or list of variables) to be eliminated:

a*b/(a + a*Cos[a/b]) // TermErsetzung[k b == a, {a, b}] 

{b/(1 + Cos[k]), a/(k (1 + Cos[k]))}

Answer 2

Rephrasing your question, what you want is to eliminate a given variable by introducing another one. There are two ways to go about it:

• The first (and easiest) is simply to express you change in variable in a rule, such as b -> a/k, and use it in a call to ReplaceAll (aka /.). This gives the following code:

  In[1]:= a*b/(a + a*Cos[a/b]) /. b -> a/k
  Out[1]= a^2/(k (a + a Cos[k]))
  

• The second way, it is covered in this Mathematica tutorial. You can use Eliminate to that aim, but it might not do exactly what you intend. For example, in your example, it will actually go further than you intended:

  In[4]:= Eliminate[{U == a*b/(a + a*Cos[a/b]) && k == a/b}, a]
  Out[4]= (-ArcCos[(b - U)/U] == k && b != 0) || (ArcCos[(b - U)/U] == k && b != 0)
  

Answer 3

In this simple example you can just use a rule. In more complex cases it might not be straighforward to generalize.

Simplify[a*b/(a + a*Cos[a/b]) /. a -> k b]

(* b/(1 + Cos[k]) *)