7 is the sole variety D for which the equation 2n − D = x2 has more than two remedies for n and x normal. Particularly, the equation 2n − 7 = x2 is named the Ramanujan–Nagell equation. The island of Ogygia was the home of the beautiful nymph named Calypso. She fell madly in love with Odysseus and promised him immortality and eternal youth