The 3 friends return to Karlikania. Their friend, the baby zero, is accosted by a mysterious x-shaped stranger, who challenges our heroes to recover his identity. Many adventures unfold, and the visitors to Karlikania master Pascal's triangle, algebraic identities, the arithemetic and geometric means, and many other challenges. Finally the mysterious stranger is released from his enchantment; an equation is solved and the mask is removed. This is mathematical storytelling at its best!

It was one of my favorite childhood books. I wish I could buy it here, in the U.S.