2 joint papers of M. A. Perles with Shelah

Kojman+Perles+Shelah, Sets in a Euclidean space which are not a countable union of convex subsets -- Israel J Math 70 (1990) 313-342
Perles+Shelah, A closed $(n+1)$-convex set in ${\bf R}^ 2$ is a union of $n^ 6$ convex sets -- Israel J Math 70 (1990) 305-312