Hello, Luca Fenzi. This looks like an interesting problem. However, it can still be improved a bit.   (1) It is recommended that at least four test cases be provided, though more are often necessary to prevent hard-coded workarounds.   (2) The Test Suite does not rigorously check the individual elements of the output vector, only their sum; that leaves it open to passing completely incorrect submissions as demonstrated in Solution 1575892.   (3) In the Problem Statement, it is highly unlikely that the output would be exactly [4, -1]; the presence of random perturbations means that the output might be [4.3, -1.2] or [3.9, -0.8], say. Although there is a suitable allowance in the Test Suite for 'random' variation in the outputs, it would still be advisable to amend the Problem Statement.   —DIV