Holy Number of Nirvana

The quest for the  was the work of Ly-Chee's life — a mathematical puzzle to which he'd devoted over 30 Toran years. Hidden amongst what he called "the sacred tales" were numbers. Solving for the relationship between the numbers would eventually lead to the so-called "".

When Ly-Chee shared the puzzle with the Seventh Doctor, the Time Lord was stumped for several days. However, he eventually saw that the riddle was "a number series puzzle with a bit of differential calculus at the end". He then concluded that the was seven. (IHP: The Enlightenment of Ly-Chee the Wise)