Like Fermat , FrÃ nicle was an amateur mathematician , but he wrote letters to famous people such as Descartes , Christiaan Huygens , Mersenne and also Fermat , who was his friend . Like Fermat , FrÃ nicle was an amateur mathematician , but he still corresponded with the likes of Descartes , Huygens , Mersenne and also Fermat , who was his personal friend .