There is a relation between rectilinear-adjacent squares such that there is a unique rectilinear path from the top-left corner of the grid down to the bottom-right corner of the grid. Each square can participate in the path no more than once. What is the relation and the path it induces?

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/s5cGk.png

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