1 (number)

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

According to the Tenth Doctor, "any number that reduce[d] to 1 when you [took] the sum of the square of its digits and continue[d] iterating it until it yield[ed] 1" was a happy number. (TV: 42)

The Twelfth Doctor once proved that $$\lim_{\theta\to 0} \frac {sin \theta} {\theta} = 1$$. (TV: The Pilot)

Block Transfer Computation made use of 1s and 0s, as it was written in binary. (AUDIO: The Enchantress of Numbers)

According to the Second Doctor, a one in 13 chance was about 7.6923 percent, odds he didn't like. (COMIC: Card Conundrum)

The Tenth Doctor believed that Dalek Caan, who had emerged as the last of the Cult of Skaro, was the one remaining Dalek in "the whole universe". (TV: Evolution of the Daleks)

The Twelfth Doctor once told Bill that "eleven plus two" was an anagram of "twelve plus one", to which Bill responded that both of these were equal to 13. (COMIC: Harvest of the Daleks)

British Summertime began in early 2005 on a Sunday morning at 01:00am when the clocks went forwards. (PROSE: The Clocks Go Forward)

The countdown towards the Ninth Doctor's eviction on Big Brother counted from 6 to 0, going through 5, 4, 3, 2, and 1. (TV: Bad Wolf) In an emergency transmission sent by the TARDIS, the Thirteenth Doctor listed 5 things that she did in any worrying situation. The first was remembering that she would get through the situation, and that darkness never prevailed. (WC: Message from the Doctor)

Behind the scenes
Though AUDIO: The Haunting establishes that 1 is itself considered a prime number in the DWU, this is a point of contention among mathematicians. It would certainly have been considered a prime, in the story's time period of the 1890s. In particular, the entry for Number published in 1890 in the 9th edition of Encyclopædia Britannica stated that every positive number was either prime or composite, and explicitly listed 1 as prime. Nowadays, in the 21st century, 1 is instead generally considered as neither prime nor composite. To further complicate real-world existence of 1 as a number, there have even been periods of time - like Ancient Greek era - where 1 was in fact not even considered a number. This included notable mathematicians like Aristotle and Euclid.

Though unremarked in PROSE: Daisy Chain, 1 is also the first and second Fibonacci number.