Which ancient Greek mathematician’s Arithmetica inspired Pierre de Fermat’s work in number theory?

The ancient Greek mathematician whose work inspired Pierre de Fermat’s contributions to number theory was Diophantus of Alexandria.12

Fermat’s interest in the field was sparked by his study of a 1621 edition of Diophantus’s treatise, the Arithmetica. While reading this text, Fermat famously recorded various remarks, generalizations, and conjectures in the margins, including the statement that became known as Fermat’s last theorem.231