In my constraint, z<=x/y. x,y,z are variables. How do I express this expression in the syntax accepted by CVX?

this is neither convex nor concave, so it can’t be done in cvx. see

I know what you mean. In the code, I’ll write it as

[x z;y 1]==semidefinite(2);

But now my program is infeasible, and I’m looking for constraints one by one, so considering whether this constraint is the problem, I wonder if there are other solutions to express this formula.

This will force [x z;y 1] to be symmetric, if I didn’t get it wrong from http://cvxr.com/cvx/doc/basics.html#set-membership. So you will have z=y, which might not be what you wanted.

I am surprised that `[x z;y 1]==semidefinite(2);`

did not produce a warning message about being non-symmetric. i don’t know what it does, but I doubt it’s anything good. It is not a magic formulation for convex alchemy to convert a non-convex constraint into a convex constraint.