@article{compref280,
   author = {Robinson,TT},
   year = {1968},
   title = {Independance of Two Nice Sets of Axioms for the Propositional Calculus},
   journal = {Journal for Symbolic Logic},
   volume = {33},
   pages = {265-270}
}

@article{compref199,
   author = {Krohn,K and Rhodes,JL},
   year = {1968},
   title = {Complexity of Finite Semi-Groups},
   journal = {Annals of Mathematics},
   volume = {88},
   pages = {128-160}
}

@book{compref264,
   author = {Popper,KR},
   year = {1968},
   title = {Logic of Scientific Discovery},
   publisher = {Hutchinson},
   address = {London}
}

@article{compref243,
   author = {Moshowitz,A},
   year = {1968},
   title = {Entropy and the Complexity of Graphs},
   journal = {Bulletin of Mathematical Biophysics},
   volume = {30},
   pages = {175-204,225-40}
}