Robert Recorde, sixteenth-century inventor of the equals sign was from Tenby. As it has been suggested, "[h]is equals sign was an invention that, while slow in becoming universally adopted, is still perhaps the most fundamental thing ever invented by a person from Wales"
William Jones, born in Anglesey in 1675, was the first recorded mathematician to use the symbol pi in its present form in 1706, though it would not achieve widespread adoption until used by famed Swiss mathematician Euler.
Magnificent as the achievements are I can't read them without thinking of the monologue in which Dr Evil says his father "would make outrageous claims like he invented the question mark".
Sometimes he would accuse chestnuts of being lazy. The sort of general malaise that only the genius possess and the insane lament.