Complex sentence: In (1), J. E. Littlewood mentions the Conjecture as an example of a mathematical claim that is easy to state but difficult to prove.Dirk Struik describes in (5) the formal analogy of the Conjecture with the Four Vertex Theorem for plane curves.
Split into simpler sentences: In (1), J. E. Littlewood mentions the conjecture as an example of a mathematical claim that is easy to state but difficult to prove. Some authors mention the purely formal analogy of the Conjecture to the Four Vertex Theorem for plane curves.