Math 310

Handy facts for the second exam

Don't forget the handy facts from the first exam!

Fermat's Little Theorem. If p is prime and (a,p) = 1, then ap-1 \medspace \underset p \medspace1

Because: (a·1)(a·2)(a·3)(a·(p-1)) \medspace \underset p \medspace 1·2·3(p-1) , and (1·2·3(p-1),p) = 1 . Same idea, looking at the a's between 1 and n-1 that are relatively prime to n (and letting f(n) be the number of them), gives

If (a,n) = 1, then af(n) \medspace \underset n \medspace1 .

f(n) = n-1 only when n is prime. Numbers n which are not prime but for which an-1 \medspace \underset n \medspace1 are called a-pseudoprimes; they are very uncommon!

One approach to calculating ak (mod n) quickly is to start with a, and repeatedly square the result (mod n), computing a1,a2,a4,a8,a16. etc. , continuing until the resulting exponent is more than half of k . ak is then the product of some subset of our list - we essentially use the powers whose exponents are part of the base 2 expansion of k.

Rings. Basic idea: find out what makes our calculations in Zn work.

A ring is a set R together with two operations +,· (which we call addition and multiplication) satisfying:

For any r,s,t R,

(0) r+s,r·s R [closure]

(1) (r+s)+t = r+(s+t) [associativity of addition]

(2) r+s = s+r [commutativity of addition]

(3) there is a 0R R with r+0R = r [additive identity]

(4) there is a -r R with r+(-r) = 0R [additive inverse]

(5) (r·s)·t = r·(s·t) [associativity of multiplication]

(6) there is a 1R R with r·1R = 1R·r = r [mutliplicative identity]

(7) r·(s+t) = r·s + r·t and (r+s)·t = r·t + s·t [distributivity]

These are the most basic properties of the integers mod n that we used repeatedly. Some others acquire special names:

A ring (R,+,·) satisfying: for every r,s R, r·s = s·r is called commutative.

A commutative ring R satisfying if rs = 0R, then r = 0R or s = 0R is called an integral domain.

A ring R satisfying if r 0R, then r·s = s·r = 1R for some s R is called a division ring.

A commutative division ring is called a field.

An element r R satisfying r 0R and r·s = 0R for some b 0R is called a zero divisor.

An element r R satisfying rs = sr = 1R for some s R is called a unit.

An idempotent is an element r R satifying r2 = r .

A nilpotent is an element r R satisfying rk = 0R for some k 1 .

Examples: The integers Z, the integers mod n Zn, the real numbers R, the complex numbers C;

If R is a ring, then the set of all polynomials with coefficients in R, denoted R[x], is a ring, where you add and multiply as you do with ``ordinary'' polynomials:

R[x] = {i = 0n rrxi : ri R} and (filling in with 0R's as needed)

i = 0n rrxi + i = 0m srxi = i = 0n (rr+si)xi and i = 0n rrxi · j = 0m srxj = k = 0n+m (i+j = kri·sj)xk

If R is a ring and n N, then the set Mn(R) of n×n matrices with entries in R is a ring, with entry-wise addition and `matrix' multiplication:

the (i,j)-th entry of (rij)·(sij) is k = 0n rik·skj

If R is commutative, then so is R[x] ; if R is an integral domain, then so is R[x]. If n 2, then Mn(R) is not commutative.

A subring is a subset S R which, using the same addition and multiplication as in R, is also a ring.

To show that S is a subring of R, we need:

(1) if s,s S, then s+s, s·s S

(2) if s S, then -s S

(3) there is something that acts like a 1 in S (this need not be 1R ! But 1R S is enough...)

The Cartesian product of two rings R,S is the set R×S = {(r,s) : r R, s S} . It is a ring, using coordinatewise addition and multiplication: (r,s)+(r,s) = (r+r,s+s) , (r,s)·(r,s) = (r·r,s·s)

Some basic facts:

A ring has only one ``zero'': if x+r = r for some R, then x = 0R

A ring has only one ``one'': if xr = r for every r, then x = 1R

Every r R has only one additive inverse: if r+x = 0R, then x = -r

-(-r) = r ; 0R·r = r·0R = 0R ; (-1R)·r = r·(-1R) = -r

Every finite integral domain is a field; this is because, for any a 0R, the function ma:R R given by ma(r) = ar is one-to-one, and so by the Pigeonhole Principle is also onto; meaning ar = 1R for some r R .

If R is finite, then every r R, r 0R, is either a zero-divisor or a unit (and can't be both!). Idea: The first time the sequence 1,r,r2,r3, repeats, we either have rn = 1 = r(rn-1) or rn = rn+k, so r(rn+k-1-rn-1) = 0 .

A unit in R×S consists of a pair (r,s) where each of r,s is a unit. (The same is true for idempotents and nilpotents.)

For n N and r R, we define n·x = x++x (add x to itself n times) and xn = x··x (multiply x by itself n times). And we define (-n)·x = (-x)++(-x) . Then we have

(n+m)·r = n·r + m·r, (nm)·r = n·(m·r), rm+n = rm·rn, rmn = (rm)n

Homomorphisms and isomorphisms

A homomorphism is a function f:R S from a ring R to a ring S satisfying:

for any r,r R , f(r+r) = f(r) + f(r) and f(r·r) = f(r) ·f(r) .

The basic idea is that it is a function that ``behaves well'' with respect to addition and multiplication.

An isomorphism is a homomorphism that is both one-to-one and onto. If there is an isomorphism from R to S, we say that R and S are isomorphic, and write R @ S . The basic idea is that isomorphic rings are ``really the same''; if we think of the function f as a way of identifying the elements of R with the elements of S, then the two notions of addition and multiplication on the two rings are identical. For example, the ring of complex numbers C is isomorphic to a ring whose elements are the Cartesian product R×R, provided we use the multiplication (a,c)·(c,d) = (ac-bd,ad+bc) . And the main point is that anything that is true of R (which depends only on its properties as a ring) is also true of anything isomorphic to R, e.g., if r R is a unit, and f is an isomorphism, then f(r) is also a unit.

The phrase ``is ismorphic to'' is an equivalence relation: the composition of two isomorphisms is an isomorphism, and the inverse of an isomorphism is an isomorphism.

A more useful example: if (m,n) = 1, then Zmn @ Zm×Zn . The isomorphism is given by

f([x]mn) = ([x]m,[x]n)

The main ingredients in the proof:

If f: R S and y: R T are ring homomorphisms, then the function w: R S×T given by w(r) = (f(r),y(r)) is also a homomorphism.

If m|n, then the function f: Zn Zm given by f([x]n) = [x]m is a homomorphism.

Together, these give that the function we want above is a homomorphism. The fact that (m,n) = 1 implies that f is one-to-one; then the Pigeonhole Principle implies that it is also onto!

The above isomorphism and induction imply that if n1,nk are pairwise relatively prime (i.e., if i j then (ni,nj) = 1), then

Zn1nk @ Zn1××Znk . This implies:

The Chinese Remainder Theorem: If n1,nk are pairwise relatively prime, then for any a1,ak N the system of equations

x ai (mod ni), i = 1,k

has a solution, and any two solutions are congruent modulo n1nk .

A solution can be found by (inductively) replacing a pair of equations x a (mod n) , x b (mod m), with a single equation x c (mod nm), by solving the equation a+nk = x = b+mj for k and j, using the Euclidean Algorithm.

Application to units and the Euler f-function:

If R is a ring, we denote the units in R by R* . E.g., Zn* = {[x]n ; (x,n) = 1}. From a fact above, we have (R×S)* = R*×S* .

f(n) = the number of units in Zn = |Zn*|; then the CRT implies that if (m,n) = 1, then f(mn) = f(m)f(m) . Induction and the fact that if p is prime f(pk) = pk-1(p-1) = pk-(the number of multiples of p) implies

If the prime factorization of n is p1a1pkak, then f(n) = [p1a1-1(p1-1)][pkak-1(pk-1)]

Groups: Three important properties of the set R* of units of a ring R:

1R R*

if x,y R*, then xy R*

if x R* then (x, by definition, has a multiplicative inverse x-1 and) x-1 R*

Together, these three properties (together with associativity of multiplication) describe what is called a group.

A group is a set G together with a single operation (denoted *) satisfying:

For any g,h,k G,

(0) g*h G [closure]

(1) g*(h*k) = (g*h)*k [associativity]

(2) there is a 1G G satisfying 1G*g = g*1G = g [identity]

(3) there is a g-1 G satisfying g-1*g = g*g-1 = 1G [inverses]

A group (G.*) which, in addition, satisfies g*h = h*g for every g,h G is called abelian. Since this is something we always expect out of addition, if we know that a group is abelian, we often write the group operation as ``+'' to help remind ourselves that the operation commutes.

Examples: Any ring (R,+,·), if we just forget about the multiplication, is an (abelian) group (R,+) .

For any ring R, the set of units (R*,·) is a group using the multiplication from the ring. [[Unsolved (I think!) question: is every group the group of units for some ring R?]]

Function composition is always associative, so one way to build many groups is to think of the elements as functions. But to have an inverse under function composition, a function needs to be both one-to-one and onto. [One-to-one is sometimes also called injective, and onto is called surjective; a function that is both injective and surjective is called bijective.] So if, for any set S, we set

G = P(S) = {f:S S : f is one-to-one and onto} ,

then G is a group under function composition; it is called the group of permutations of S. If S is the finite set {1,2,,n}, then we denote the group by Sn, the symmetric group on n letters. By counting the number of bijections from a set with n elements to itself, we find that Sn has n! elements.

The set of rigid motions of the plane, that is, the functions f:R2 R2 satisfying dist(f(x),f(y)) = dist(x,y) for every x,y R2, is a group under function composition, since the composition or inverse of rigid motions are rigid motions. More generally, for any geometric object T (like a triangle, or square, or regular pentagon, or...), the set of rigid motions f which take T to itself form a group, the group Symm(T) of symmetries of T. For example, for T = a square, Symm(T) consists of the identity, three rotations about the center of the square (with rotation angles p/2,p,, and 3p/2), and four reflections (through two lines which go through opposite corners of the square, and two lines which go through the centers of opposite sides).

G = Aff(R) = {f(x) = ax+b : a 0} , the set of linear, non-constant functions from R to R, form a group under function composition, since the composition of two linear functions is linear, and the inverse of a linear function is linear. It is called the affine group of R. This is an example of a subgroup of P(R):

A subgroup H of G is a subset H G which, using the same group operation as G, is a group in its own right. As with subrings, this basically means that:

(1) If h,k H, then h*k H

(2) If h H, then h-1 H

(3) 1G H .

Condition (3) really need not be checked (so long as H ), since, for any h H, (2) guarantees that h-1 H, and so (1) implies that h*h-1 = 1G H .

For example, for the symmetries of a polygon T in the plane, since a symmetry must take the corners of T (called its vertices) to the corners, each symmetry can be thought of as a permutation of the vertices. So Symm(T) can be thought of (this can be made precise, using the notion of isomorphism below) as a subgroup of the group of symmetries of the set of vertices of T.

As with rings, some basic facts about groups are true:

There is only one ``one'' in a group; if x G satisfies x*y = y for some y G, then x = 1G

Every g G has only one inverse: if g*h = 1G, then h = g-1

(g-1)-1 = g for every g G

(gh)-1 = h-1g-1

Homomorphisms and isomorphisms: Just as with rings, again, we have the notion of functions between groups which ``respect'' the group operations:

A homomorphism is a function f:G H from groups G to H which satisfies:

for every g1,g2 G, f(g1*g2) = f(g1)*f(g2)

No other condition is required, since this implies that

f(1G) = 1H ,as well as f(g-1) = (f(g))-1 .

An isomorphism is a homomorphism that is also one-to-one and onto. If there is an isomorphism from G to H, we say that G and H are isomorphic. As with rings, the idea is that ismorphic groups are really the ``same''; the function is a way of identifying elements so that the two groups are identical (as groups!). For example, the group Aff(R) can be thought of as R×R (i.e., the pair of coefficients of the linear function), but with the group multiplication given by (by working out what the coeffiicients of the composition are!) (a,b)*(c,d) = (ac,ad+b) .

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