Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Discrete mathematics, continuous mathematics, and probability resit / 3 / b / i

The prime numbers $2,3,5$ form the lower antichain; the nonprime elements $1,4$ form the upper antichain. In particular, $1$ is not a prime number. Each lower element is below each upper element, and no third element can lie strictly between them. Therefore the six cover relations are $2,3,5\prec 1,4$, represented by the following Hasse diagram. Upward movement represents increasing order; crossing lines do not add elements.