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July 17th, 2012, 10:55 AM  #1 
Senior Member Joined: Sep 2011 Posts: 140 Thanks: 0  The counting of psylow groups
Ok it says that for a group with order G, G=60, it has 6 5sylow subgroups and 10 3sylow subgroups, and 12,5 2sylow subgroups. Ok i cant see why this is the case. Because , and say that it has a 5sylow group, then by sylow's third theorem, then there are {1,6,11,......56} by congruence with 1mod(5) right? then the only divisible with the order of G=60 is 1 and 6, therefore 2 5sylow subgroups. Ok maybe i am not doing it right, but isn't this is how you count it? i just dont see how its 6 5sylow groups. Also how is there a two possibility of a 2sylow for a order 60 group? can anyone explain it to me? thank you 
July 17th, 2012, 10:35 PM  #2 
Senior Member Joined: Mar 2012 Posts: 294 Thanks: 88  Re: The counting of psylow groups
a 5sylow subgroup of a group G of order 60 has order 5. of course, these all intersect in the identity, so if k is the number of 5sylow subgroups of G, these account for 4k+1 elements of G. by the third sylow theorem, k = 1 (mod 5), and k divides 60, so either k = 1, or k = 6. note that if k = 6, the 5sylow subgroups only account for 25 elements of G, leaving 35 elements of other orders. let's show that both scenarios actually occur. for a single 5sylow subgroup, consider (Z60,+), the integers mod 60 under addition modulo 60. the sole subgroup of order 5 is: {0,12,24,36,48} for 6 5sylow subgroups, consider A5. there are 24 (= 4!) 5cycles in S5, and all of these are in A5. explicitly we have: <(1 2 3 4 5)> <(1 2 3 5 4)> <(1 2 4 3 5)> <(1 2 4 5 3)> <(1 2 5 3 4)> <(1 2 5 4 3)>, which are 6 distinct subgroups of order 5 in A5. of course a 2sylow subgroup is possible. such a subgroup has (for a group of order 60) order 4. for a group of order 60, we can have 1,3 5, or 15 of them (these being the odd divisors of 60/4 = 15). again, using our two examples, we see Z60 has just one subgroup of order 4: {0,15,30,45} while A4 has the following: {e, (1 2)(3 4), (1 3)(2 4), (1 4)(2 3)} {e, (1 2)(3 5), (1 3)(2 5), (1 5)(2 3)} {e, (1 2)(4 5), (1 4)(2 5), (1 5)(2 4)} {e, (1 3)(4 5), (1 4)(3 5), (1 5)(3 4)} {e, (2 3)(4 5), (2 4)(3 5), (2 5)(3 4)} which accounts for all possible subgroups of order 4 in A5 (two double 2cycles which don't move the same 4 elements result in a 3cycle when multiplied), so we have 5 2sylow subgroups. you might want to hazard a guess as to how many 2sylow subgroups D30 (the dihedral group of order 60) has. 

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counting, groups, psylow 
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