Number

Numbers were studied in mathematics.

Certain numbers had certain properties. For example, the Tenth Doctor once stated that any number that "reduce[d] to 1 when you take the sum of the square of its digits, and continue[d] iterating it until it yield[ed] 1," was a happy number. Moreover, if such number was also prime, it was called a happy prime. (TV: 42)

In mathematics, i was an imaginary number that was the value of the square root of negative one. (AUDIO: Iterations of I)