Mathematics was the study of numbers.
Romulus and Remus Sylvest were mathematical geniuses. (TV: The Twin Dilemma) Peter Hutchings was a professor of mathematics at the University of Cambridge. (PROSE: Timewyrm: Revelation) Danny Pink taught mathematics at Coal Hill School. (TV: Into the Dalek) Spending time in 1138 Essex, the Twelfth Doctor mentioned he'd provided the citizens with "top-notch maths tuition in a fun but relevant way", among other things. (TV: The Magician's Apprentice) Before becoming a Renegade, the Master taught mathematics and computer science at the Prydonian Academy for a time. (PROSE: CIA File Extracts)
Some of the companions of the Doctor studied or taught mathematics. Jackson Lake was a mathematics teacher. (TV: The Next Doctor) Alistair Gordon Lethbridge-Stewart taught mathematics at Brendon Public School. (TV: Mawdryn Undead) Adric was awarded a gold star badge for mathematical excellence. (TV: Full Circle) Zoe Heriot introduced herself to the Second Doctor and Jamie McCrimmon as "astrophysicist, pure mathematics major", to which her colleague, Dr Gemma Corwyn, added "with honours". (TV: The Wheel in Space)
Types of mathematics
The Tenth Doctor mentioned recreational mathematics, which involved the study of happy prime numbers, numbers which are both happy and prime. He seemed surprised it wasn't a regularly studied course. (TV: 42)