In 1878 [George Arnold, the father of M. C. Escher[,] left Japan. He did not find it difficult to get work back in Holland. After some fruitful visits to colleagues at the Ministry of Transport, he applied for the post of District Engineer in Maastrich. After his stay in Japan, and as the result of an inheritance, his financial position had improved. So he started to look for a wife.
'However, Catholic Maastrich was not suitable in this respect,' he writes in his autobiography. 'I did know some gifted and attractive women and girls in the Protestant circle but these by no means fitted the equation w = 1/2m + 10, in which 'w' is the right age for a girl and 'm' represents the man's age.
F. H. BOOL et al.
Escher: The Complete Graphic Work, London & New York, 1982, p. 10