F. A. Valentine in his book ([1],
p. 177, Problem 6.5) suggests that a sufficient condition for a nonempty compact and
connected subset S of E2 to have a kernel consisting of a single point is that each
triple of points of S can see via S a unique point of S. The authors show that this
condition is sufficient if S is any subset of a topological linear space which contains a
noncollinear triple.