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).
The 332nd lost 25 bombers.). At least 25 bombers being escorted by the Tuskegee Airmen over Europe during World War II were shot down by enemy aircraft, according to a new Air Force report. The report contradicts the legend that the famed black aviators never lost a plane to fire from enemy aircraft.