In 1638 Descartes wrote a letter to Mersenne where he talks about how the number $$D=3^2⋅7^2⋅11^2⋅13^2⋅22021$$ would be an odd perfect number if we mistakingly assume that $22021$ is prime. My question is, do we know what method Descartes used when he found his number and does anyone know where I can find a copy of the letter that he sent to Mersenne in November of 1638?
I've tried several searches online, and at our local academic library, but have had no luck. Any help would be greatly appreciated!