7 (number)

7 was a prime number. (AUDIO: The Haunting)

According to Jo Grant, three sevens were equal to 21. (TV: The Claws of Axos)

7 was not divisible by 2. (PROSE: And Then Again)

According to the Seventh Doctor, the Holy Number of Nirvana was 7. (COMIC: The Enlightenment of Ly-Chee the Wise)