The induced path transit function
J(u,v) in a graph consists of the set of all vertices lying on any induced path between the vertices
u and
v. A transit function
J satisfies monotone axiom if
x,y
J(u,v) implies
J(x,y)
J(u,v). A transit function
J is
said to satisfy the Peano axiom if, for any
u,v,w
V,x
J(v,w),
y
J(u,x), there is a
z
J(u,v) such that
y
J(w,z). These two axioms are equivalent for the induced path transit function of a graph. Planar graphs for which the induced path transit function satisfies the monotone axiom are characterized by forbidden induced subgraphs.