Logspace reductions and directed reachability
A logspace many-one reduction computes f with x∈A exactly when f(x)∈B, using a read-only input, O(log n) work tape and write-only output.
Key knowledge
A logspace many-one reduction computes f with x∈A exactly when f(x)∈B, using a read-only input, O(log n) work tape and write-only output. Directed s–t reachability is NL-complete under these reductions: bounded path guessing uses logarithmic space, and each NL computation reduces to its polynomial-size configuration graph.
Conditions and scope
- The reduction halts for every input; its output need not fit into the work tape and is polynomial in length.
- To compose reductions or simulate B, regenerate requested output symbols instead of storing the whole intermediate output. Graph vertices and edges have an explicit finite encoding.
Sources · 1
References
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Evidence and review
Lecture 20 slides 6–8: transducer/reduction definitions, symbol-regeneration argument and PATH completeness proof.
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