Greene's theorem (http://ac.els-cdn.com/0097316576900789/1-s2.0-0097316576900789-main.pdf?_tid=3b618666-d514-11e5-b014-00000aacb362&acdnat=1455672037_585aef34d8038d78798742ec5858d8f6) and the Greene-Kleitman theorem (http://ac.els-cdn.com/0097316576900777/1-s2.0-0097316576900777-main.pdf?_tid=69f23e94-d514-11e5-8ffd-00000aacb35f&acdnat=1455672115_9d28b3c1302190aec361c8aad6f7da12) are remarkably deep theorems that hold for any finite partially ordered set.