How many times can a shape be completely surrounded by copies of itself without being able to tile the entire plane, i.e., what is the maximum (finite) Heesch number?
Heesch's Problem
See also
Heesch NumberExplore with Wolfram|Alpha
References
Eppstein, D. "Heesch's Problem." http://www.ics.uci.edu/~eppstein/junkyard/heesch/.Referenced on Wolfram|Alpha
Heesch's ProblemCite this as:
Weisstein, Eric W. "Heesch's Problem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HeeschsProblem.html