assume(x^2 == y) doesn't work

I have the following code:

syms x y;
assume(x^2 == y);
assume(x*y == y*x);

t = isequaln(x^2, y);
s = isequaln(x*y, y*x);

The value for t is 0, so it doesn't seem like isequaln(x^2, y) is working correctly. (The value for s is 1, so it seems like assume(x*y == y*x) is working fine.) I've also tried simplify(x^2 + y) and this just returns x^2 + y. So there must be some exception with assumptions that involve the same variable twice?

Any help would be appreciated. Thanks.

Answer 1

[Tommy]

From the docs,

"assume is not additive. Instead, it automatically deletes all previous assumptions on the variables in condition."

Try assumeAlso.

(edit) Also, take a look at this post.

Here is a simple test of isequaln and isAlways:

>> syms x y
>> assume(x^2 == y)
>> assumeAlso(x*y == y*x)
>> assumptions
 
ans =
 
[ x*y == x*y, x^2 == y]
 
>> isequaln(x*y,y*x)

ans =

  logical

   1
   
   % (I'm guessing this is true because multiplication of complex numbers is commutative,
   %  not because of the above assumption.)

>> isequaln(x*x,y)

ans =

  logical

   0

>> isAlways(x*x==y)

ans =

  logical

   1