Gas sensing as an inverse problem . The forward problem is to predict the response of the sensor array m given the gas composition x. The solution is to evaluate the adsorption model m = a(x). The inverse problem arises in gas sensing: we wish to predict the gas composition x given the sensor response m using the adsorption model m = a(x). While the forward problem always has a unique solution, the inverse problem may have a unique solution, no solution, or many solutions.