There are problems that just end up being really hard and by definition the only problems that come to my desk are the ones that nobody else can solve.
1 is not prime, by definition. 2 is an unnatural prime, 4 is an unnatural prime, and 6 is an unnatural prime. All other natural primes cannot be unnatural primes.
Hash, x. There is no definition for this word - nobody knows what hash is. Famous, adj. Conspicuously miserable. Dictionary, n. A malevolent literary device for cramping the growth of a language and making it hard and inelastic. This dictionary, however, is a most useful work.