Frederik was the name of three sons of King Frederik IV of Denmark. 1. Frederik (1701-1702) was the son with his first wife Luise. He died young 2. Frederik (1704-1705) was the son with his second wife Elisabeth. Also died young. 3. Frederik (1726-1727) was the son with his third wife Anna. Also died young.
Frederik (1599-1599) was the oldest son of King Christian IV of Denmark and his wife Anna. He died young. His younger brother (1609-1670) had the same name and later became King Frederik III of Denmark.