What are the paper IDs of papers presented in conferences has a homepage starts with "http://www.informatik.uni-trier.de/~ley/db/conf/"?
homepage starts with "http://www.informatik.uni-trier.de/~ley/db/conf/" refers to HomePage LIKE 'http://www.informatik.uni-trier.de/~ley/db/conf/%'
SELECT T1.Id FROM Paper AS T1 INNER JOIN Conference AS T2 ON T1.ConferenceId = T2.Id WHERE T2.HomePage LIKE 'http://www.informatik.uni-trier.de/~ley/db/conf/%'