Question

In Mathematica, how can I simplify expressions like a == b || a == -b into a^2 = b^2? Every function that I have tried (including Reduce, Simplify, and FullSimplify) does not do it.

Note that I want this to work for an arbitrary (polynomial) expressions a and b. As another example,

a == b || a == -b || a == i b || a == -i b

(for imaginary i) and

a^2 == b^2 || a^2 == -b^2

should both be simplified to a^4 == b^4.

Note: the solution should work at the logical level so as not to harm other unrelated logical cases. For example,

a == b || a == -b || c == d

should become

a^2 == b^2 || c == d.

Answer 1

The Boolean expression can be converted to the algebraic form as follows:

In[18]:= (a == b || a == -b || a == I b || a == -I b) /. {Or -> Times,
    Equal -> Subtract} // Expand

Out[18]= a^4 - b^4

---

EDIT

In response to making the transformation leave out parts in other variables, one can write Or transformation function, and use in Simplify:

In[41]:= Clear[OrCombine];
OrCombine[Verbatim[Or][e__Equal]] := Module[{g},
  g = GatherBy[{e}, Variables[Subtract @@ #] &];
  Apply[Or, 
   Function[
     h, ((Times @@ (h /. {Equal -> Subtract})) // Expand) == 0] /@ g]
  ]

In[43]:= OrCombine[(a == b || a == -b || a == I b || a == -I b || 
   c == d)]

Out[43]= a^4 - b^4 == 0 || c - d == 0

Alternatively:

In[40]:= Simplify[(a == b || a == -b || a == I b || a == -I b || 
   c == d), TransformationFunctions -> {Automatic, OrCombine}]

Out[40]= a^4 == b^4 || c == d