# Understanding Converse of Lagrange's theorem!

Lagrange's theorem states that for any finite group $G$, the order (number of elements) of every subgroup $H$ of $G$ divides the order of $G$ .......(1)

The converse of Lagrange's theorem is if $x$ divides order of $G$ ,then there exists a sub group of order $x$. ...... (2)

If my statement is $p\to q$ then converse is $q\to p$.

i couldn't understand how converse of Lagrange's theorem comes...please explain with this definition of converse : $p\to q$ then converse is $q\to p$

• (I'm sorry if this question is uninteresting But please help!) – Cloud JR Aug 12 '18 at 8:59
• – mrs Aug 12 '18 at 9:03
• I don't want counter example! i want to know how (2) is converse of (1) – Cloud JR Aug 12 '18 at 9:05
• I have given an answer for how to formulate the converse of the Lagrange's theorem. However, the other answers as for the counterexample to disprove the converse statement are also correct! – Aniruddha Deshmukh Aug 12 '18 at 9:05

Actually, you need to write the Lagrange theorem in $p \rightarrow q$ form. So, the Lagrange theorem is actually,

If $H$ is a subgroup of $G$, then order of $H$ divides order of $G$.

Here $p:$ $H$ is a subgroup of $G$

$q;$ Order of $H$ divides order of $G$.

Here, actually the existence of $H$ is in the hypothesis ($p$) and the number $m$ is actually the order of the subgroup which divides order of $G$.

So, the converse will be

If a number $m$ divides order of $G$, then there is a subgroup of order $m$.

I hope this gives you insights.

• It seems that the OP gets trouble in understanding a logical statement. – mrs Aug 12 '18 at 9:07
• @ResidentDementor yes you are correct – Cloud JR Aug 12 '18 at 9:08
• Yes. Actually he does! Which is why I think that he should have made the tag of "logic" instead of "abstract algebra". – Aniruddha Deshmukh Aug 12 '18 at 9:08
• well i change the tags.. thanks for your suggestion – Cloud JR Aug 12 '18 at 9:10
• So Lagrange's theorem can be restated as If there exists a subgroup H of order m ,then m | order of G.. Is this correct? – Cloud JR Aug 12 '18 at 9:13

What is exactly your question? Do you want a counter example of the converse of Lagrange's theorem?

Such a counter example is follwing: The alternating group $A4$ has 12 elements, but doesn't contain a subgroup of order 6.

• Better,you made this as comment – mrs Aug 12 '18 at 9:02
• You are of course right. I'd like write this as a comment, but I don't have 50 reputation to do so. – mathlettuce Aug 12 '18 at 9:05
• I don't want counter example! i want to know how (2) is converse of (1) – Cloud JR Aug 12 '18 at 9:05

Take the group $A_4$, then group of all even permutations of $4$ elements. $o(A_4)$ is $12$, but it doesn't have any subgroup of order $6$. Please check it !!

• I don't want counter example! i want to know how (2) is converse of (1) – Cloud JR Aug 12 '18 at 9:05
• The order of $A_4$ is $12$ – leibnewtz Aug 12 '18 at 9:20
• Yes, you're right @leibnewtz. My mistake !! – Anik Bhowmick Aug 12 '18 at 9:21