Savitch deterministic space simulation
For a space-constructible s(n)≥log n, NSPACE(s(n))⊆DSPACE(s(n)²).
Key knowledge
For a space-constructible s(n)≥log n, NSPACE(s(n))⊆DSPACE(s(n)²). A recursive reachability test on the computation's configuration graph stores one midpoint configuration per level: O(s(n)) levels each need O(s(n)) workspace. Consequently PSPACE=NPSPACE and NL⊆DSPACE(log²n).
Conditions and scope
- Use the read-only input/workspace convention and deciders of the space-class record.
- The simulation bound squares space; it does not assert L=NL, P=NP, or a polynomial-time simulation of every polynomial-space computation.
Sources · 3
References
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Massachusetts Institute of Technology2 sources
- CS6810 (Spring 2026 posting), Lecture 8: Space complexity and logspaceEshan Chattopadhyay / Cornell UniversityCourse notes · PDF
Evidence and review
Lecture 18 slide 4 theorem and midpoint/configuration-count proof for bounds ≥n; Lecture 20 slide 4 directly checks the logspace case. Cornell Lecture 8 printed page 1 states the general space-constructible ≥log n form.
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