  These notes contain, among others, a proof that the average running time of
an easy solution to the satisfiability problem for propositional calculus is,
under some reasonable assumptions, linear (with constant 2) in the size of the
input. Moreover, some suggestions are made about criteria for tractability of
complex algorithms. In particular, it is argued that the distribution of
probability on the whole input space of an algorithm constitutes an
non-negligible factor in estimating whether the algorithm is tractable or not.
