seven is the one quantity D for which the equation 2n − D = x2 has over two alternatives for n and x pure. In particular, the equation 2n − 7 = x2 is called the Ramanujan–Nagell equation.The island of Ogygia was the house of The attractive nymph named Calypso. She fell madly in like with Odysseus and promised him immortality and Everlasting y