Welsh mathematicians

Numerous mathematicians who have made contributions to the history of mathematics have hailed from the country of Wales.

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" ([1]).
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.
Bertrand Russell, perhaps the most influential British thinker of the twentieth century, though more properly a philosopher than a mathematician, was of English descent but born in Monmouthshire.