The sign (or parity) of a permutation is a group-homomorphism from to $latex S_2 [^1] that appears in the definition of the determinant. Proving that the sign defines a group-homomorphism is not difficult, but the (very short) standard proof[^2] is fairly unintuitive. Therefore
This post describes a more visual proof of the fact that the sign of a permutation is a homomorphism and gives some interesting facts relating to the sign.
Permutations – a visual description
Let be a permutation. Then we can write explicitly using two line notation, for example is the permutation that sends 1 to 2, and 2 to 1, 3 to 5 and so on.
The parity, or sign of a permutation is defined as where is the non-identity element in (it is easy to see that has two elements, one of which is the identity, denoted by ) and . Basically looks at whether the number of inversions in is even or odd. A nice way of visualising permuations is by drawing which elements get sent where. In this way, the permutation corresponds to the following picture:
Crossings and the sign
The number of lines that cross1 gives the number of so that and this number is . Whether or not this number is even or odd determines the sign. In this case, we see immediately that . Deforming a single line can change the number of crossings, but (provided that each crossing is proper and no more than two lines cross at a point) doing so introduces/removes an even number of crossings so the sign is well-defined.
The graphical representation (called picture for this post) also tells us that the sign of the identity is 1 and that inverting an element does not change the sign (just flip the picture).
Compositions of permutations can be drawn graphically :
The idea is that if we “deform” the black lines into the blue lines, we can only get rid of even numbers of crossings in the process. If you look at the picture above long enough this should be clear.
From this fact, it follows that is a homomorphism. For if we draw the pictures of and over each other, we obtain a picture of . The total number of crossings is the number of crossings in plus the number of crossings in . Calling the number of crossings in this picture of (and similarly for and we have that which finishes the proof.
A more formal way of phrasing this is that if is an inversion in (i.e. taking then ), then either it is an inversion in or is an inversion in , but not both. If both are inversions, or neither of them is, then is not inversion in . Hence the parity of the number of inversions in is the sum (modulo 2) of the number of inversions in and .
Signs in the wild
Apart from being used in the formula for calculating determinants, the sign of a permutation is also useful in other contexts. For example, for every we can define as the alternating group over elements. Because is a homomorphism it follows that is normal subgroup. For it can be shown that is the only nontrivial2 normal subgroup of .
Permutations also are used to define orientations of objects in differential geometry and algebraic topology. Here it is useful to say that the triangle with vertices is times the triangle with vertices , where . The vertices of both triangles are of course the same, but they are treated as different objects depending on how their vertices are ordered.
Lastly, looking at a permutation in one line notation is fairly clear that by observing that one line crosses all of the other ones. Knowing that elements with same cycle type are conjugate and that is a homomorphism, this gives the following formula if is composed of disjoint cycles of length :