Has the same relation to NL as FNP does to NP.
Defined by [AJ93], who also showed that if NL = UL, then FNL is contained in #L.
Has the same relation to FNL as P/poly does to P.
The class of problems reducible in L to the problem of whether an undirected graph has a unique connected component.
See [AG00] for more information.
The corresponding class for directed graphs equals NL. On the other hand, none of that class's corresponding search problems are obviously FNL-hard.
Has the same relation to L as UP does to P.
The problem of reachability in directed planar graphs lies in UL [SES05].
If UL = NL, then FNL is contained in #L [AJ93].