Π

The mathematical concept of Pi (3.14159...) was the ratio of any circle's circumference to its diameter.

In the Tomb of Rassilon on Gallifrey a death trap existed to guard Rassilon, where stepping so far into the room would cause lightning to strike down upon the intruders. Pi was the only way to reach across the room without being destroyed. (DW: The Five Doctors)

The square root of Pi is (approximately) 1.772453850905516027298167483341. The Doctor used that sequence to understand the intelligence of the Midnight Entity. (DW: Midnight)