I want to know the answer to the following question:
What is the title and name of the ruler whose son Chopin was friends with?
All the background I'm provided with is that "During this period, Fryderyk was sometimes invited to the Belweder Palace as playmate to the son of the ruler of Russian Poland, Grand Duke Constantine; he played the piano for the Duke and composed a march for him.". Is that enough to answer the question?