Mathematicians have studied knots for the last 150 years or so, or ever since topology started to crystallize as a mathematical discipline. One classical problem is to show that the left-handed and right-handed knots really are different. To keep themselves honest, mathematicians stick the free ends of each knot together, to form a loop which holds the knot captive. When overhand knots are made into loops, the resulting configuration has a threefold symmetry, like the outline of three leaves. Hence the names left-handed trefoil and right-handed trefoil for the closed form of these knots.
Left- and right-handed trefoils
Topologists consider two knots to be the same if one can be deformedto match the other. The statement ``Left- and right-handed trefoil are different''means to a topologist that no matter how the string is stretched or twisted in three-dimensional space, thereis no way to change one into the other without tearing the string.
How do you prove mathematically that something can't be done?The usual way in topology is to figure out some quantity thatcan be calculated froma configuration and such that when the configurationis changed following the rules (here: without tearing the string),that quantity does not change. Such a quantity is called aninvariant. If you can find an invariant which givesdifferent values for the left- and right-handed trefoil, that willprove that one cannot be deformed to the other.