Katy Perry launched her career as a Christian pop star named Katy Hudson in 2001, which gave no indication that by 2008 she would take the pop world by storm with the debauched, raucous and cheeky album One of the Boys that featured feisty, electro-pop-rock songs named “I Kissed a Girl” and “Ur So Gay.” Perry’s surprising re-entry into the world of pop also came through in her songwriting, which she’s been doing for other artists since around the time of her career change. Hear some of the catchy, anthemic tracks Perry has written for everyone from Nicki Minaj to Selena Gomez.