A predecessor to Sudoku:
Scientific American Magazine
November 1959
Volume 201, Issue 5
Emi Kasai bases on a Mutually Orthogonal Latin squares (MOLS)
https://en.wikipedia.org/wiki/Mutually_orthogonal_Latin_squares#Euler's_conjecture_and_disproof
http://martin-gardner.org/SciAm12.html
https://www.scientificamerican.com/issue/sa/1959/11-01/
