I need the Proof:

a, b are two elements of a group G such that ab=ba.

If o(a)=m & o(b)=n and m,n are relatively prime to each other,then

o(ab)=mn.

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- Apr 29th 2010, 02:17 PMmanikorder of an element of a group
I need the Proof:

a, b are two elements of a group G such that ab=ba.

If o(a)=m & o(b)=n and m,n are relatively prime to each other,then

o(ab)=mn. - Apr 29th 2010, 03:07 PMGnomeSain
It's obvious that .

To show that mn is the smallest such number, note that the order of ab must be a multiple of both a and b. Depending on how strictly your teacher grades, you may want to explicitly justify this statement. Since m and n are relatively prime the smallest such number is their LCM, which is mn. - Apr 29th 2010, 06:54 PMBruno J.
Yes, and that's what the problem essentially is about!

Suppose (where the last equality is justified because commute). Use the division algorithm to write , with and . Then . Now raise this expression to the power , eliminating from the left side; you obtain . Now we know has the same order as (prove it, using ), and therefore we must have , which implies . Similarily we must have . Therefore is divisible both by and therefore divisible by . - Apr 30th 2010, 12:55 PMGnomeSain