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).
â¦ The most lethal jet fighter of ALL time was undoubtedly the F-4 Phantom ! -With it's use throughout Vietnam by the USAF, the US Navy, and the US Marines and in various wars with the Israeli Air Force the Phantom quite possibly caused more destruction than any 2 other jet types ever.