-  Authors: H. Buhrman, S. Fenner, L. Fortnow and D. van
Melkebeek.
 -  Reference: In Proceedings of the 17th International Symposium 
     on Theoretical Aspects of Computer Science, pages 407-418, 2000.
 -  Earlier version:
-  H. Buhrman, S. Fenner, L. Fortnow and D. van Melkebeek.
     Optimal Proof Systems and 
     Sparse Sets. In
      Proceedings of 17th International Symposium on Theoretical 
     Aspects of Computer Science, 2000.
 
 
Abstract
We exhibit a relativized world where the class of sparse NP sets has no 
complete sets. This gives a relativized world where no optimal 
proof systems exist. 
We also examine under what reductions the class of sparse NP sets can 
have complete sets. We show a close connection between these issues and 
reductions from sparse to tally sets. 
We also consider the question as to whether the sparse NP sets have a 
computable enumeration.