  A free-form Sudoku puzzle is a square arrangement of m times m cells such
that the cells are partitioned into m subsets (called blocks) of equal
cardinality. The goal of the puzzle is to place integers 1,...,m in the cells
such that the numbers in every row, column and block are distinct. Represent
each cell by a vertex and add edges between two vertices exactly when the
corresponding cells, according to the rules, must contain different numbers.
This yields the associated free-form Sudoku graph. This article studies the
eigenvalues of free-form Sudoku graphs, most notably integrality. Further, we
analyze the evolution of eigenvalues and eigenspaces of such graphs when the
associated puzzle is subjected to a "blow up" operation, scaling the cell grid
including its block partition.
