7 is the only range D for which the equation 2n − D = x2 has a lot more than two solutions for n and x natural. In particular, the equation 2n − seven = x2 is known as the Ramanujan–Nagell equation. The island of Ogygia was the house of https://7-day-pill-organizers42862.national-wiki.com/1832457/a_review_of_7_day_pill_organizers