# Groups 1. Definition 1 A Group G is a set with an operation which satisfies the following: e a = a e = e. a a 1 = a 1 a = e.

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1 Groups 1 1 Introduction to Groups Definition 1 A Group G is a set with an operation which satisfies the following: 1. there is an identity element e G, such that for every a G e a = a e = e 2. every element has an inverse, i.e. if a G there exists an element a 1 G a a 1 = a 1 a = e 3. the group is closed under the operation, i.e. if a G and b G then a b G. 4. the associativity rule is satisfied: i.e. for all a, b, c G What does this mean? A group consists of two things: 1. a set of elements a (b c) = (a b) c 2. an operation which acts upon two elements in the set to return another element. This operation must satisfy the properties given above. the set can contain a finite or infinite number of elements the set must contain an identity. There can only be one identity. The identity depends on the operation. Example: For the real numbers, 0 is the identity for addition, but 1 is the identity for multiplication. Every element in the set must have an inverse in the set too (depending on the operation) Example: For the real numbers, taking the number 5, under addition its inverse is 5, but under multiplication it is 1 5.

2 The operator may not give you an element which is not in the set, so if a, b G and then it is not a group. a b = c c / G Associativity just checks that the order which you apply the operation does not matter. Note that we do not care about the following: a b = b a In many groups (especially those of matrices) this is not true, but that s okay. If this is true, then we say the operation is commutative. As long as then the group is closed under the operation. a b G and b a G A group whose operation is commutative is called Abelian. 1.1 Examples Examples are core to understanding the concept of groups. I shall take some set and operation pairings (S, ) and see if they are groups by checking the required properties G 1 = (Z, +) Here the set is all the integers Z, under the operation of addition +. Taking each property in turn: Identity: What number can you add to 5 to get 5? More mathematically x + 5 = 5, solve for x. Ans: x = 0. So we can see that for addition the identity is zero, since for every a Z a + 0 = 0 + a = a Inverse: What number can you add to 5 to get the identity 0? That is we want to solve x + 5 = 0 for x. Ans: x = 5. So under addition, 5 is the inverse of 5. Extending this, you may say that for every a Z there is an inverse a Z so that a + ( a) = ( a) + a = 0 Closure: If you add an integer to an integer, do you always get an integer? Yes! It is enough to state this fact, and conclude that when a, b G 1 then a + b G 1. Associativity: The operation of addition is associative since i.e. the order you add integers is irrelevant. Since all four properties are true, then G 1 a group. a + (b + c) = a + b + c = (a + b) + c

3 1.2 G 2 = (R, ) Here we take the set of all real numbers with the operation of multiplication. Identity: What number can you multiply by 5 to get 5? Ans: 1. So for addition, the identity is one, since for every a R a 1 = 1 a = a 1 Inverse: What number can you multiply by 5 to get the identity 1? Ans:. So under 5 multiplication, 1 is the inverse of 5. Extending this, can we say that for every a R there is 5 an inverse 1 R so that 5 a 1 a = 1 a a = 1? No! The element 0 does not have an inverse! The fraction 1 0 is not defined. Since this property is false, G 2 is not a group. How can we make this a group? Since the only problem element is 0, try the following: 1.3 G 3 = (R \ {0}, ) Here we take the set of all real numbers, but excluding the number 0, with the operation of multiplication. Identity: As above, the identity is 1. Inverse: Is it true that for every a R there is an inverse 1 a R \ 0 so that Yes! a 1 a = 1 a a = 1? Closure: If you multiply two (non-zero) real numbers, do you always get a (non-zero) real number? Yes! It is enough to state this fact, and conclude that when a, b G 3 then a b G 3. Associativity: The operation of addition is associative since a (b c) = a b c = (a b) c since the order you multiply real numbers is irrelevant. Since all four properties are true, G 3 is a group.

4 1.4 G 4 = (M 2 (R), +) Here the set is all the 2 2 matrices whose entries are real numbers, under the operation of addition +. So every element a M 2 (R) is of the form: ( ) a1,1 a a = 1,2 a 2,1 a 2,2 where a 1,1, a 1,2, a 2,1, a 2,2 R. Addition of two matrices is defined as: ( ) ( ) ( ) a1,1 a 1,2 b1,1 b + 1,2 a1,1 + b = 1,1 a 1,2 + b 1,2 a 2,1 a 2,2 b 2,1 b 2,2 a 2,1 + b 2,1 a 2,2 + b 2,2 ( ) Taking each property in turn: Identity: a M 2 (R). Take Then it is easy to see that e = ( ) 0 0 M (R) a + e = e + a = a Inverse: For every a M 2 (R) there is an inverse a M 2 (R) of the form ( ) a1,1 a a = 1,2 a 2,1 a 2,2 a + ( a) = ( a) + a = 0 Closure: If you add two real numbers, you always get an real number (since (R, +) is a group). Adding two matrices is in effect, adding their components. The components are real, so their sum is real It is enough to state this, and conclude that when a, b G 4 then a + b G 4. Associativity: The operation of addition is associative since addition of real numbers is associative (as (R, +) is a group). i.e. the order you add integers is irrelevant. a + (b + c) = a + b + c = (a + b) + c Since all four properties are true, then G 4 is a group. 1.5 G 5 = (M 2 (R) \ {0}, ) Here the set is all the 2 2 matrices whose entries are real numbers, excluding the all zero matrix, i.e. ( ) = 0 0

5 under the operation of multiplication. Multiplication of two matrices is defined as: ( ) ( ) ( ) a1,1 a 1,2 b1,1 b 1,2 a1,1 b = 1,1 + a 1,2 b 2,1 a 1,1 b 2,1 + a 1,2 b 2,2 a 2,1 a 2,2 b 2,1 b 2,2 a 2,1 b 1,1 + a 2,2 b 2,1 a 2,1 b 2,1 + a 2,2 b 2,2 ( ) Taking each property in turn: Identity: a G 5. Take Then it is easy to see that e = ( ) 1 0 G a + e = e + a = a Inverse: For every a G 5 is there an inverse a 1 G 5 of the form ( ) a 1 1 a2,2 a = 1,2 a 1,1 a 2,2 a 1,2 a 2,1 a 2,1 a 1,1 such that a a 1 = a 1 a = 0? Warning! But what if a 1,1 a 2,2 a 1,2 a 2,1 = det(a) = 0? Then we have a divide-by-zero! Which means not every element in G 5 has an inverse, so G 5 is not a group! 1.6 G 6 = ({a M 2 (R) where det(a) 0}, ) Here the set is all the 2 2 matrices whose entries are real numbers with non-zero determinant, under the operation of multiplication. Note that the zero matrix has zero determinant, so is excluded anyway. Taking each property in turn: Identity: As above, the identity is e = ( ) 1 0 G Inverse: For every a G 6 there is an inverse a 1 G 6 of the form a 1 = 1 ( ) a2,2 a 1,2 det(a) a 2,1 a 1,1 so that a a 1 = a 1 a = 0 Here it is impossible that det(a) = 0, so we are safe.

6 Closure: If you multiply two real numbers, you always get an real number (since (R \ {0}, ) is a group). Adding two matrices is in effect, adding their components. The components are real, so their sum is real It is enough to state this, and conclude that when a, b G 6 then a b G 6. Associativity: The operation of addition is associative since addition of real numbers is associative (as (R, +) is a group). i.e. the order you add integers is irrelevant. a + (b + c) = a + b + c = (a + b) + c Since all four properties are true, then G 6 a group. More examples of groups use things other than numbers or matrices. However the rules to be checked are the exact same. 1.7 Permutations 1.8 Rotations 2 More Non-Groups The following are not groups. Can you see why? (Z, ) the integers under multiplication (Z, ) the integers under subtraction (Z, ) the integers under division (R, ) the integers under subtraction (R, ) the integers under division

7 3 Answers to More Non-Groups (Z, ) no element has an inverse except the identity 1. (Z, ) associativity rule broken (Z, ) not closed, associativity rule broken (R, ) associativity rule broken (R, ) not closed ( 1 / R), associativity rule broken 0

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