Good, I. J. and Gaskins, R. A. (1980). Density estimation and bump-hunting by the penalized likelihood method exemplified by scattering and meteorite data. Journal of the American Statistical Association, 75, 42--56.
Leonard, T. (1978). Density estimation, stochastic processes and prior information. Journal of the Royal Statistical Society. Series B (Methodological), 40, 113--146.