There are two mathematical problems known as the Einstein problem, only one of which was devised by Einstein. The problems are as follows:
1. Is there a single shape that will tile a plane aperiodically (see aperiodic tiling)? An answer of "no" would imply the existence of a decision method for the domino problem. The problem, which is named because of the German translation (ein = "one," stein = "stone") and was not an invention of the famous scientist, remains unsolved.
· There are 5 houses (along a street) in 5 different colors: blue,
green, red, white, and yellow.
· In each house lives a person of a different nationality: Briton, Dane, German, Norwegian, and Swede.
· These 5 owners drink a certain beverage: beer, coffee, milk, tea, or water; smoke a certain brand of cigar: Blue Master, Dunhill, Pall Mall, Prince, or blend; and keep a certain type of pet: cat, bird, dog, fish, or horse.
· No owners have the same pet, smoke the same brand of cigar, or drink the same beverage.
· The Briton lives in a red house. The Swede keeps dogs as pets. The Dane drinks tea. The green house is on the left of the white house (next to it). The green house owner drinks coffee. The person who smokes pall Mall rears birds. The owner of the yellow house smokes Dunhill. The man living in the house right in the center drinks milk. The Norwegian lives in the first house. The man who smokes blend lives next to the one who keeps cats. The man who keeps horses lives next to the man who smokes Dunhill. The owner who smokes Blue Master drinks beer. The German smokes Prince. The Norwegian lives next to the blue house. The man who smokes blend has a neighbor who drinks water.
The question is: Who keeps the fish?