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Unformatted text preview: Homework #5 Solutions p 83, #16. In order to find a chain h a 1 i h a 2 i h a n i of subgroups of Z 240 with n as large as possible, we start at the top with a n = 1 so that h a n i = Z 240 . In general, given h a i i we will choose h a i 1 i to be the largest proper subgroup of h a i i . We will make repeated use of the fundamental theorem of cyclic groups which tells us that a cyclic group of order m has a unique subgroup of order d for any d  m . The largest proper subgroup of Z 240 has size 120 and is h 2 i . Since  2  = 120, the largest proper subgroup of h 2 i has size 60 and is h 4 i . Since  4  = 60, the largest proper subgroup of h 4 i has size 30 and is h 8 i . Since  8  = 30, the largest proper subgroup of h 8 i has order 15 and is h 16 i . Since  16  = 15, the largest possible subgroup of h 16 i has order 5 and is h 48. Finally, since  48  = 5 is prime, the only proper subgroup of h 48 i is h i . Therefore, we have produced the maximal chain h i h 48 i h 16 i h 8 i h 4 i h 2 i h 1 i which has length 7. Notice that the chain h i h 120 i h 60 i h 30 i h 15 i h 5 i h 1 i also has length 7, but is produced in the opposite way, i.e. by starting with h i and at each stage choosing h a i +1 i as the smallest subgroup containing h a i i . p 83, # 20. Let x G . Since x 35 = e , we know that  x  = 1 , 5 , 7 or 35. Since  G  = 35, if G contains an element x of order 35, then G = h x i as desired. On the other hand, if G contains an element x of order 5 and and element y of order 7, then, since G is abelian ( xy ) 35 = x 35 y 35 = ee = e so that the order k of xy divides 35. That is,  xy  = 5 , 7 or 35. If  xy  = 5 then e = ( xy ) 5 = x 5 y 5 = ey 5 = y 5 which means that 7 =  y  divides 5, a contradiction. Likewise, we have a similar problem if  xy  = 7. It follows that  xy  = 35, and as above that G is cyclic. So, what we need to do is show that G must have an element of order 5 and an element of order 7. We argue by contradiction. If G has no elements of order 5 then every nonidentity element of G has order 7. That is, there are 34 elements in G or order 7. However, by the corollary to Theorem 4.4, the number of elements in G of order 7 is divisible by (7) = 6, and 34 is not divisible by 6. Likewise, if G had no element of order 7 then G would contain 34 elements of order 5, and this number would have to be divisible by (5) = 4, which is also impossible. It follows that G must have at least one element of order 5 and at least one of order 7. As we pointed out above, this forcesof order 7....
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This note was uploaded on 02/29/2012 for the course MATH 3362 taught by Professor Ryandaileda during the Fall '10 term at Trinity University.
 Fall '10
 RyanDaileda

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