We define the (locally valid) unresponsive subspace B ⊂ X of a sensor array associated with operating point x op as the directions in composition space such that, for small changes in the gas composition in these directions, the sensor response remains approximately constant at m op . Thus, for a unit vector ∆x ∈ B, a(x op + ∆x) ≈ a(x op ) with a small number. Invoking the linear approximation ã(x) in eqn. 2, the local unresponsive subspace is given by the null space of the Jacobian matrix J(x op ):