Possible Duplicate:
Replacing composite variables by a single variable
In order to simplify an given expression with several variables, I want to replace a subterm by a new variable. I tried to do the following with Mathematica:
myexpression := <variables, sums, products etc. here, something like a^2+b^2>
subterm := <variables, sums, products etc. here, something like a^2>
myexpression /. subterm -> newvar
For simple examples this works fine, but for my case it just returns the unchanged expression myexpression.
Can I be sure that the subterm can't be found in myexpression? Is there any good formal definition for "a subterm $t$ can't be found in expression $e$"? (of corse you could replace the first $t$ in $e -t + t = e$, but this is obviously not what we want...)
The code above is a simplified example, the Mathematica code for my specific problem looks like this:
Quit[]
Ra[i_] := ((1 + P[i])/2)*Pa[i] + ((1 - P[i])/2)*Pb[i]
Rb[i_] := ((1 - P[i])/2)*Pa[i] + ((1 + P[i])/2)*Pb[i]
Rab[i_] := Pab[i]
PXa := Ra[1]*Ra[2] + Ra[1]*Rab[2] + Rab[1]*Ra[2]
PXab := Ra[1]*Rb[2] + Rb[1]*Ra[2] + Rab[1]*Rab[2]
DX := PXa + ((PXab)/2) - (1 + P0)/2
DY[i_] := Pa[i] + ((Pab[i])/2) - (1 + (P0/P[i]))/2
DX /. DY[1] -> DY1
DX /. DY[2] -> DY2
