In Soviet Mathematics, the Soviet Union of two sets is similar to the mathematical union of two sets.
In Mathematics, taking the union of sets A and B takes all the elements from both sets and gives a set C containing all the elements of both sets.
Taking the Soviet Union of two sets takes all the elements from both sets and gives them to the glorious nation-state of Soviet Russia.
In mathematical notation, the difference is clear:
Another, similar type of Soviet Mathematics is the Soviet Intersection.
In Mathematics, taking the intersection of set A and B gives you a set C containing only the elements that are in set A and B.
The Soviet Intersection however, takes a difference in ideologies and gives you a great bloody wall down the middle of the German capital. (Berlin Wall)
The Soviet Union was the first in the long line of mathematical tools invented specifically to serve communism. In 1925 when communism was still in its infancy and so inclined to make strange babbling noises, The Man of Steel realised that using mathematics from the western world could be seen as a dangerous espousement of western ideals. As a result, he commissioned the mathematician Vladimir Steklov (lit. transl. "Vladimir Lover of Steaks") to construct a new Mathematics based entirely on the principles of communism. Unfortunately Vladimir died of mad cow disease the following year, but not before he had paved the way for Soviet Mathematics by writing down the definitions shown above.
Another early theorist of Soviet Mathematics was the great Nikolai Ivanovich Lobachevsky. His main contribution was the improvement of the Soviet Mathematics curriculum of Moscow State University. By introducing the policy of stating that mathematical operations should be performed "For the glory of our Soviet Motherland!" (i.e.: "For the glory of Mother Russia, we will carry the two!"), efficiency was increased by 10,000% (2% in western mathematics). Unfortunately, much of Lobachevsky's work was discredited after it was discovered that he had plagarized his thesis paper from a colleague in Dnipropetrovsk.
Work on Soviet Mathematics was continued by Stalin himself, and his concepts of Soviet Integration and Soviet Differentiation of Mother Russia from Western scum went on to form the cornerstone of his governing policy, until this cornerstone was nicked to rebuild Cuba. He also introduced the idea that variables could be airbrushed out of an equation, and their existence denied. However, when it was discovered that the letter x was not suceptible to the Siberian climate, this practice was curtailed.