Polikarpov Po-2
The Polikarpov Po-2 (also U-2) served as a general-purpose Soviet biplane, nicknamed Kukuruznik (Russian: Кукурузник, from Russian "kukuruza" (кукуруза) for maize; thus, "maize duster" or "crop duster"), NATO reporting name "Mule". The reliable, uncomplicated concept of the Po-2's design made it an ideal training aircraft, as well as doubling as a low-cost ground attack, aerial reconnaissance, psychological warfare and liaison aircraft during war, proved to be one of the most versatile light combat types to be built in the Soviet Union. As of 1978 it remained in production for a longer period of time than any other Soviet-era aircraft.
It is one of the most produced aircraft, and may be the most produced biplane with production possibly as high as 30,000 Po-2s built between 1928 and 1959. However, production figures for Polikarpov U-2 and Po-2 bombers and trainers combined are between 20,000 and 30,000. with production ending as early as 1952. Correct figures are hard to come by since low-rate production by small repair shops and air clubs likely continued until 1959.
Unitary group
In mathematics, the unitary group of degree n, denoted U(n), is the group of n × n unitary matrices, with the group operation that of matrix multiplication. The unitary group is a subgroup of the general linear group GL(n, C). Hyperorthogonal group is an archaic name for the unitary group, especially over finite fields.
In the simple case n = 1, the group U(1) corresponds to the circle group, consisting of all complex numbers with absolute value 1 under multiplication. All the unitary groups contain copies of this group.
The unitary group U(n) is a real Lie group of dimension n2. The Lie algebra of U(n) consists of n × n skew-Hermitian matrices, with the Lie bracket given by the commutator.
The general unitary group (also called the group of unitary similitudes) consists of all matrices A such that A∗A is a nonzero multiple of the identity matrix, and is just the product of the unitary group with the group of all positive multiples of the identity matrix.
Properties
Since the determinant of a unitary matrix is a complex number with norm 
, the determinant gives a group homomorphism
U2 (disambiguation)
U2 is an Irish rock band.
U2 or U-2 may also refer to:
Military
Railway
Other uses
SM U-2 (Germany)
SM U-2 was a German U-boat built for the Imperial German Navy. Only one of the type, sometimes called German Type U 2 submarine, was built. U-2 was ordered from Kaiserliche Werft of Danzig on 4 March 1906, launched on 18 June 1908, and commissioned into the Imperial German Navy on 18 July 1908. She conducted no war patrols and spent World War I as a training platform.
After Germany's surrender, she was decommissioned on 19 February 1919 and sold for shipbreaking to Stinnes on 3 February 1920.
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