7 is the one number D for which the equation 2n − D = x2 has over two remedies for n and x natural. Specifically, the equation 2n − seven = x2 is called the Ramanujan–Nagell equation. The island of Ogygia was the home of The gorgeous nymph named Calypso. https://senioraidtools.com/10-easy-mobility-exercises-for-seniors-to-stay-active-and-independent/