Sustainability of US public debt: Estimatingsmoothing spline regressions ∗ Alfred Greiner G¨oran KauermannDepartment of Business Administration and EconomicsBielefeld University, P.O. Box 10013133501 Bielefeld, Germany Abstract In this paper we analyze how the primary surplus to GDP ratio in the USreacts to variations in the public debt-GDP ratio. In contrast to earlier studies weperform non-parametric and semi-parametric estimations. Our results show thatthe response of the primary surplus to GDP ratio is a positive nonlinear functionof the debt-GDP ratio. Further, our estimations demonstrate that the coeﬃcientgiving the response of the surplus ratio to a change in the debt ratio declines overtime when we assume a linear model with time dependent coeﬃcients.