# Find the centralizer of $(123)$ in $S_6$.

Find the centralizer of $$(123)$$ in $$S_6$$.

Is there any software to calculate $$C_{(123)}$$ where $$C_{(123)}$$ denotes centralizer of $$(123)$$ in $$S_6$$.

Can anyone say how to write the code to find centralizer of $$(123)$$ in $$S_6$$?

Can it be done using SageMath?

I checked that it can be done by SageMath but it only lists the generators of the centralizer of $$(123)$$ in $$S_6$$.

Is there any code to list all the elements of the centralizer of $$(123)$$ in $$S_6$$?

• Is it $S_5$ or $S_6$? – Parcly Taxel Aug 3 at 9:13
• @ParclyTaxel; edited the question – Math_Freak Aug 3 at 9:18
• If you want the code to list all the elements, just add code at the end which calculates the subgroup explicitly from the generators you are given. – James Aug 3 at 10:08
• @James;can you kindly give the code – Math_Freak Aug 3 at 10:15
• Please remove one of the tags and insert the "GAP" tag. – Marc Bogaerts Aug 3 at 20:28

## 1 Answer

The centralizer of $$a = (1,2,3)$$ is rather simple to describe. First assemble the permutations of $$\{4,5,6\}$$. There are $$6$$ of them. Then precede each of these with the permutations $$a^0,a,a^3$$ which gives you the $$18$$ elements of the centralizer. Here is the code in GAP:

gap> c1 := SymmetricGroup([4..6]);
Sym( [ 4 .. 6 ] )
gap> c2 := Group((1,2,3));
Group([ (1,2,3) ])
gap> c := Group(Union(c1,c2));
Group([ (), (5,6), (4,5), (4,5,6), (4,6,5), (4,6), (1,2,3), (1,3,2) ])


But Gap has the direct command 'Centralizer':

gap> S6 := SymmetricGroup(6);
Sym( [ 1 .. 6 ] )
gap> Centralizer(S6,(1,2,3)) = c;
true


GAP tag failed to add (more than 5 tags)

• How do you know that there will be 18 elements – Math_Freak Aug 4 at 2:52
• Because there are $6$ permutations of $\{4,5,6\}$ and $3$ elements of the group generated by $(1,2,3)$. The centralizer consists of the products of an element of the first set with an element of the second set, giving $6 \times 3 = 18$ elements. – Marc Bogaerts Aug 4 at 4:33
• My simple question is:: Why will the centralizer consist of "products of an element of the first set with an element of the second set" What is the mathematical explanation for this? – Math_Freak Aug 4 at 6:01
• It are the only elements that commute with $(1,2,3)$, any other element of $S_6$, like e.g. $(3,4,5)$ or $(1,2)(3,6)$ does not commute. – Marc Bogaerts Aug 4 at 7:17
• That does the give the required proof – Math_Freak Aug 4 at 12:24