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).
Published September 13, 2016. FoxNews.com. Iran threatened to shoot down two US Navy surveillance aircraft flying close to Iranian territory in the Persian Gulf over the weekend, the latest in a series of recent provocations between Iran and the US military in the region, three US defense officials with knowledge of the incident told Fox News.