1. Every planar map is 4-colorable—1: Discharging;Appel;Illinois Journal of Mathematics,1977
2. Every planar map is 4-colorable—2: Reducibility;Appel;Illinois Journal of Mathematics,1977
3. Perceptions, PP, and the polynomial hierarchy;Beigel;Computational Complexity,1994
4. Probabilistic polynomial time is closed under parity reductions;Beigel;Information Processing Letters,1991
5. A. Borodin, A. Demers, Some comments on functional self-reducibility and the NP hierarchy, Technical Report TR 76-284, Cornell Department of Computer Science, Ithaca, NY, July 1976