Gradient boosting decision tree in the prediction of NO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si159.svg" display="inline" id="d1e1697"><mml:msub><mml:mrow /><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math> emission of waste incineration