Daniel P. Bovet Pierluigi Crescenzi Riccardo Silvestri Complexity Classes and Sparse Oracles. 382-390 1995 50 J. Comput. Syst. Sci. 3 db/journals/jcss/jcss50.html#BovetCS95 http://dx.doi.org/10.1006/jcss.1995.1030