7 is the sole number D for which the equation 2n − D = x2 has more than two alternatives for n and x natural. Particularly, the equation 2n − 7 = x2 is recognized as the Ramanujan–Nagell equation.The island of Ogygia was the house of the beautiful nymph named Calypso. She fell madly in adore with Odysseus and promised him immortality and Ever