E xamples o of Groups The integers mod n under addition G=LN= the integers modulo N=(0.N-1) the group operator is"+ modular addition the integers modulo n are closed under addition the identity is 0 the inverse of x is-x addition is associative addition is commutative(so the group is abelian
Examples of Groups The integers mod N under addition G = Z+ N = the integers modulo N = {0 … N-1} the group operator is “+”, modular addition • the integers modulo N are closed under addition • the identity is 0 • the inverse of x is -x • addition is associative • addition is commutative (so the group is abelian)
E xamples o of Groups The integers mod p under multiplication G=Z'n=the non-zero integers modulo p=(1.p-1) the group operator is "*>, modular multiplication the integers modulo p are closed under multiplication this is so because if GCD(x, p)=l and gCd(y p)=1 then gCd(xy,p)=1 the identity is 1 the inverse of x is from euclids algorithm ux+ vp=1=GCD(x,p) SOX=u alsoxl=u=xp-2 multiplication is associative multiplication is commutative(so the group is abelian)
Examples of Groups The integers mod p under multiplication G = Z* p = the non-zero integers modulo p = {1 … p-1} the group operator is “*”, modular multiplication • the integers modulo p are closed under multiplication: this is so because if GCD(x, p) =1 and GCD(y,p) = 1 then GCD(xy,p) = 1 • the identity is 1 • the inverse of x is from Euclid’s algorithm: ux + vp = 1 = GCD(x,p) so x-1 = u also x-1 = u = xp-2 • multiplication is associative • multiplication is commutative (so the group is abelian)
Examples of groups ZN: the multiplicative group mod N G=Z n the positive integers modulo n relatively prime to N the group operator is"x>, modular multiplication the integers modulo n are closed under multiplication this is so because if GCD(x, n)=l and GCD( n)=1 then gCd(xy, n)=1 · the identity is the inverse of x is from Euclids algorithm Ux+VN=1=GCD(X, N) So X=uX(N-) multiplication is associative multiplication is commutative(so the group is abelian)
Examples of Groups Z* N : the multiplicative group mod N G = Z* N = the positive integers modulo N relatively prime to N the group operator is “*”, modular multiplication • the integers modulo N are closed under multiplication: this is so because if GCD(x, N) =1 and GCD(y,N) = 1 then GCD(xy,N) = 1 • the identity is 1 • the inverse of x is from Euclid’s algorithm: ux + vN = 1 = GCD(x,N) so x-1 = u (= x f(N)-1 ) • multiplication is associative • multiplication is commutative (so the group is abelian)
Examples of a non-abelian group GL(2), 2 by 2 non-singular real matrices under matrix multiplication a b GL(2)= ad-bC=0 if a and b are non-singular so is aB the identity is I=o1] a c al/(ad-bc) matrIx multiplication is associative matrix multiplication is not commutative
Examples of a non-abelian group GL(2), 2 by 2 non-singular real matrices under matrix multiplication • if A and B are non-singular, so is AB • the identity is I = [ ] • = /(ad-bc) • matrix multiplication is associative • matrix multiplication is not commutative GL(2) = {[ ], ad-bc = 0} a b c d 1 0 0 1 [ ] a b c d -1 [ ] d -b -c a
S subgroups (H, @) is a subgroup of(G, @)if H is a subset of g (H, @ is a group
Subgroups (H,@) is a subgroup of (G,@) if: • H is a subset of G • (H,@) is a group