Update: After installing CVX 3.0beta, Build 1175, it works as expected.
I would like to write 1/(x(1)*x(2)) <= w.
According to the reference guide, this should be possible with prod_inv(x).
But I get: "Undefined function or variable 'n'.":
variables x(s) w
x >= 1
x <= 2
prod_inv(x) <= w
Setting s=1 works fine.