INSTANCE: Graph , length function , and a pair of vertices s,t in V.
SOLUTION: Two vertex disjoint paths in G connecting s and t, i.e. two sequences of vertices and such that , , , , and are included in E, and for any i, and are included in E.
MEASURE: The longest of the two paths, i.e. .
Comment: Approximable within 2 also if the paths should be edge disjoint instead of vertex disjoint. Variation in which the graph is directed and we look for two vertex (edge) disjoint directed paths is also approximable within 2, and is not approximable within for any .