In this paper we establish c-bit semi-external graph algorithms, - i.e., algorithms which need only a constant number c of bits per vertex in the internal memory. In this setting, we obtain new trade-offs between time and space for I/O efficient LTL model checking. First, we design a c-bit semi-external algorithm for depth-first search. To achieve a low internal memory consumption, we construct a RAM-efficient perfect hash function from the vertex set stored on disk. We give a similar algorithm for double depth-first search, which checks for presence of accepting cycles and thus solves the LTL model checking problem. The I/O complexity of the search itself is proportional to the time for scanning the search space. For on-the-fly model checking we apply iterative-deepening strategy known from bounded model checking.
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