Double Bar. The double bar symbol is used to denote certain kinds of norms in mathematics (e.g., or). It is also used to denote parallel lines, as in, and in an older notation for the covariant derivative. In this work, the notation or is reserved for matrix and function norms, respectively.
The F-4 Phantom II remained in use by the U.S. in the reconnaissance and Wild Weasel (Suppression of Enemy Air Defenses) roles in the 1991 Gulf War, finally leaving service in 1996. It was also the only aircraft used by both U.S. flight demonstration teams: the USAF Thunderbirds (F-4E) and the US Navy Blue Angels (F-4J).
Price: $111,200,000.00. Hours To Get Around The World: 40.36 hours. The F-1117 was strictly a ground-attack aircraft during the Gulf War of 1991. It also took part in the conflict in Yugoslavia in 1999, where one of the planes was shot down by a surface-to-air missile and became the only Nighthawk to be lost in combat. The U.S. Air Force officially retired the F-117 in April 2008.