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7 is the only number D for which the equation 2 n − D = x 2 has more than two solutions for n and x natural. In particular, the equation 2 n − 7 = x 2 is known as the Ramanujan–Nagell equation. 7 is one of seven numbers in the positive definite quadratic integer matrix representative of all odd numbers: {1, 3, 5, 7, 11, 15, 33}. [19] [20]
7 S, Z: Presumed abbreviation of septem, Latin for 7. 9.5 X̷: Scribal abbreviation, an x with a slash through it. Likewise, IX̷ represented 8.5 11 O: Presumed abbreviation of onze, French for 11. 40 F: Presumed abbreviation of English forty. 70 S: Also could stand for 7, with the same derivation. 80 R: 90 N: Presumed abbreviation of nonaginta ...