For two integers a and b, gcd(a,b) is the greatest natural number which is a divisor of both a and b. To compute the gcd, Euclid came up with an excellent algorithm, unsurprisingly known as Euclid's Algorithm.

Definition: Let a and b be integers (both not 0). A positive integer d is the greatest common divisor (GCD) of a and b if:
1) d|a and d|b
2) If c|a and c|b, then c|d.

Definition: If x and y are positive integers, x is a divisor of y, denoted x|y, if xq=y for some integer q.

Theorem: Let a and b be integers (not both 0), then a greatest common divisor d of a and b exists and is unique. Moreover there exists integers x and y such that d=ax+by.

Proof: We must first show that and integer d is a common divisor of a and b. Let a and b be integers, not both 0. Let S={n>0|n=ax+by for integers x and y}. Notice that a=a(1)+b(0) and b=a(0)+b(1), so a and b are elements of S. Further, -a=a(-1)+b(0) and -b=a(0)+b(-1), so -a and -b are in S. So S contains at least 1 positive integer.

By the Well Ordering Principle, S has a least element, namely d. Since d is in S, there are integers x and y such that d=ax+by. Now, by the Division Algorithm, to divide by d there must be integers q and r such that a=d*q+r where 0<=r<d. Thus r=a-d*q=a-(ax+by)q=a(1-x)+b(-qy). Therefore r is in S. Since d is the least element of S, we conclude that r=0. Then a=dq and d|a. Similarily, d|b. Thus d is a common divisor of a and b.

Now we must prove that d is the greatest common divisor of a and b. Assume that c is a common divisor of a and b. Then a=cu and b=cv for integers u and v. So d=ax+by=(cu)x+(cv)y=c(ux+vy). Thus c|d. Hence d is the greatest common divisor of a and b.

We must now show that d is unique. Suppose d1 and d are greatest common divisors of a and b. Then d1|a and d1|b and if c|a and c|b, then c|d1. But d is also a greatest common divisor, so d|a and d|b, so d|d1. Similarily d1|d. Thus d1=ds and d=d1t for integers s and t. So d=d1t=dst which implies d=d(st). So 0=d(st)-d=d(st-1) which implies either d=0 or st-1=0. But d cannot be 0 since d is a positive integer, so st-1=0 which implies st=1. Thus s=t=1 or s=t=-1. So d1=d(1) and d1=d. Q.E.D.

A generalised proof of the above theorem comes from ring theory. It is useful both in that it applies to objects other than natural numbers, and as it puts the natural number case into context and shows why the above proof works at a deeper level. This is of course just an example of the abstract process of mathematical abstraction.

Anyway, the translation of the theorem into ring theory goes like this :

Suppose we have a ring R which is a principle ideal domain and which contains unity (an identity element under the multiplicitive group, written 1). And suppose we have a and b elements of R. Then there exists an element d in R s.t. d|a, d|b, and if another element c in R is such that c|a and c|b, then c|d. Also, there exist e, f in R s.t. ae + bf = d.

The proof relies on the idea of ideals. First, we consider I := aR + bR. This can quite easily be seen to be an ideal in R. Now, since we said R is a principle ideal domain, this means I is a principle ideal - i.e. there exists d in R s.t. I = dR. We show d satisfies the conditions in the theorem.

Now, aR is a subset of I, and 1 is in R, so a is in I, which means there is some element g of R s.t. dg = a i.e. d|a. Similarly, d|b.

And suppose there exists a c in R s.t. c|a and c|b, and consider the ideal J := cR. We have a, b are in J, and since ideals are closed under multiplication by members of the ring, so are aR and bR. And then using the closure of ideals under addition, we have I = aR + bR is in J, which means d is in J, which means c|d

So d is indeed the gcd of a and b, and the main part of the theorem now comes easily from the way we constructed I - since dR = I = aR + bR, there exist e,f in R s.t. d = ae + bf. Q.E.D.

What the above doesn't prove is the uniqueness of d. That's because we have to assume rather more for that, which reduces the generality of the theorem. For example, the above theorem applies to polynomials over a field, say Q (the rationals). But though a gcd of x+1 and 2x2-2 is x+1, so is 2(x+1), and each divides the other since 2 and 1/2 are both in Q. But that doesn't make the concept of gcd any less powerful here. For example, it applies also to matrix polynomials, which has consequences in the field of linear algebra

Lastly, as a demonstration of the power of abstraction, an example of how our generalised version sheds new light on the area of the original theorem. Two integers are coprime iff their gcd is 1. So we already knew that if a and b are coprime, we can find e and f s.t. ae + bf = 1. But note that from our proof I is now 1R i.e. I = Z itself, the integers. So in fact we can find e and f s.t. ae + bf = g for any integer g at all.

In a computer algebra setting, the greatest common divisor is necessary to make sense of fractions, whether to work with rational numbers or ratios of polynomials. Generally a canonical form will require common factors in the numerator and denominator to be cancelled. For instance, the expressions

-8/6, 4/-3, -(1+(1/3)), -1*(12/9), 2/3 - 2
all mean the same thing, which would be preferable to write as -4/3 (that is, a single fraction of the form z/n, z an integer, n natural, gcd(z,n)=1).

## Some useful properties of the gcd

Suppose we are working with a suitable ring R (for instance, the integers, or the rational polynomials). Then the 'greatest' (in the sense of magnitude for numbers, or degree for polynomials) divisor of an element r is clearly itself (1.r=r). Furthermore, for any element r of R, 0.r=0 so r is a divisor of 0. Hence a first observation:
For any r, gcd(r,0) = r.
Next up, it's completely trivial that if c is the gcd of a and b, it is the gcd of b and a. That is,
gcd(a,b)=gcd(b,a).
Letting c be any divisor, there must be α and β such that a=αc and b=βc (otherwise, c isn't a divisor!). So we observe that a - b = αc - βc =(α-β)c. That is,
If c divides a and c divides b, then c divides a-b.
Then, since division can be thought of as repeated subtraction, we deduce
If c divides a and c divides b, and if a≥b, then c divides the remainder of a/b. (for non-zero b).

## Integer GCDs

Equipped with just these four simple facts, you might be able to determine a process for finding the greatest divisor of two integers (note that this is defined to be a natural number.) If you can't, the work has conveniently already been done by Euclid, some 2 millenia ago. Despite its age, Euclid's algorithm remains the best method for this task, and the so called extended euclid algorithm even allows you to express the gcd in terms of the original integers, as dialectic's writeup above describes.

## Polynomial GCDs

To prove that expertise in this area has in fact progressed in the past two thousand years, the rest of this writeup considers the problem of finding the gcd of two polynomials (in a single variable). This is equivalent to finding their common roots, meaning that gcd calculations can be applied to solving systems of equations.

### Euclidean Techniques

A natural first approach is to refine Euclid's algorithm as devised for numbers to work on polynomials. It is indeed possible to create such an algorithm. However, one of two problems arises. Either you are forced to work with fractional coefficients (awkward) or a fraction-free approach is employed by rescaling- which causes the size of coefficients in intermediate expressions to skyrocket (again awkward). Non-euclidean techniques can be devised which call for a euclidean gcd algorithm only in circumstances where these two pitfalls can be avoided. But before discussing these, there is an approach along Euclidean lines which works somewhat better than a naive re-scaling version of Euclid's algorithm.

#### GCDs, Resultants, and the Sylvester Matrix

First, some definitions. For polynomials P = anXn + ... + a0 and Q = bmXm + ... + b0, whose roots are the sets αi and βj respectively, we define the resultant of P and Q as
r(P,Q) = Πi=1:nΠj=1:mji)
(That is, the product of all possible differences of roots)
Then the Sylvester Matrix of P and Q is an (m+n)X(m+n) square matrix generated by m copies of the coefficients of P and n copies of those of Q, shifted right each row and padded by zeros:
```an  an-1  ...  a0 0  ...  0
0   an   ...   a1 a0 ...  0
.      .                  .
.          .              .
.              .          .
0  0    ...    0 an  ... a0
bm  bm-1  ...  b0 0  ...  0
0   bm   ...   b1 b0 ...  0
.      .                  .
.          .              .
.              .          .
0  0    ...    0 bm  ... b0
```
It is a remarkable result that the determinant of this matrix is precisely the resultant of P and Q. Note that the resultant will be zero iff P and Q have a common root- in which case, they have a non-trivial greatest common divisor.

Hence, gaussian elimination can be applied to the Sylvester Matrix, analagous to Euclid's algorithm. If the result is non-zero, the polynomials are co-prime. If a result of zero is obtained, then there is a gcd of interest- with a bit more work, an algorithm can keep track of the cancellations that occur during elimination and in this way the common factors determined. Finally, a system of rescalings exists which allows for fraction-free calculation whilst generating coefficients with predictable common factors, which may therefore be efficiently cancelled. This entire technique is encapsulated in the Dodgson-Bareiss algorithm, and it represents the most efficient way to find the gcd along euclidean lines.

### Non-Euclidean techniques- modular GCD calculation

We have seen that polynomial gcd calculation is possible in a fraction-free manner, at the price of intermediate expression swell, which, although reduced by the Bareiss algorithm, can still be horrific. A useful trick would be to bound the size of those intermediate expressions, and this is generally accomplished by the use of modular methods- working modulo a small value such that all expressions fall in a range 0...n or -p...p say.

#### Two (de)motivating Examples

Sadly, the simplification power of modular mathematics can often erase interesting or important information. There is a further complication with gcds, which is that coefficients of the answer may be greater than any and all of the coefficients present in your original polynomials. Here are some examples that demonstrate these concerns directly.
Problem 1: Consider P = x3 + x2 - x -1 and Q = x4 + x3 + x + 1. Here all the coefficients are 0 or 1. However, when you factorise P and Q you observe P = (x+1)2(x-1) and Q = (x+1)2(x2 - x - 1). So their gcd is (x+1)2 which expands as x2 + 2x + 1- we have obtained a larger coefficient. Examples of this form can be created to generate an arbitrarily large coefficient.
Problem 2: Let P = x-3 and Q = x + 2. Then these are clearly coprime- so their gcd is 1. Yet working modulo 5, Q = P so we have a non-trivial gcd of x + 2, of greater degree than the 'true' gcd.

#### Solutions to these problems

The first issue, of unexpectedly large coefficients, can be fairly easily resolved- there is an upper bound, the Landau-Mignotte bound, on the absolute value of coefficients in the gcd- generated by a fairly ugly formula involving the degrees of the polynomials and their coefficients. For a precise description, see the literature referenced at the end of this writeup; its existence is sufficient for the discussion that follows.

The second problem needs more work to resolve, but is easy to describe. Given polynomials P and Q, we observe (without proof) that

• degree( gcd((P mod n),(Q mod n)) ) ≥ degree( gcd(P,Q) )
• Equality holds in the above only if gcd((P mod n),(Q mod n)) = (gcd(P,Q)) mod n. That is, if the modular image of the gcd is the gcd of the modular images.
• The above will fail only if n divides the resultant of P/G, Q/G, where G is the true gcd of P and Q.
• The resultant is finite, so has only finitely many prime factors. Hence, if we chose n to be prime, then only finitely many choices of n will be 'bad'.
A 'bad' gcd will be immediately apparent- it won't actually divide both P and Q! On the other hand, a gcd found by modular means has degree at least that of the true gcd, so if it turns out to be a divisor, it is the gcd. So if we just keep generating gcds working modulo a prime, eventually we'll exhaust the bad primes and uncover the true gcd.

Note that in the two algorithms offered below, a gcd calculation is required! This seems circular- but we already have a working yet potentially unwieldy gcd algorithm from the Dodgson-Bareiss approach, which, modulo a prime, won't be so unwieldy after all.

#### Large prime modular GCD algorithm

Pick p a prime greater than twice the LM (Landau-Mignotte) bound.
Calculate Qp = gcd(Ap,Bp) (where Xp denotes X mod p)
Rewrite Qp over the integers such that its coefficients fall in the range -p/2 to p/2 (that is, add or subtract p from each coefficient that falls outside the LM bound)
If Qp divides A and divides B, Qp is the true gcd.

#### Many small primes modular GCD algorithm

The above single-prime technique could still yield large coefficients if the LM bound is high; and we only determine if it was a 'good' prime at the very end. Using the Chinese remainder theorem, it is possible to build up to the LM bound in stages, discarding any unlucky primes along the way. By using successive small primes, no modular gcd calculation will be very difficult, and there need not be many such iterations- knowing the answer mod m and n, the Chinese remainder theorem yields an answer mod mn.

```GCD(A,B)
Define LM= Landau-Mignotte bound of A,B
Define p=firstPrime(A,B)
p should be chosen so as not to divide
the leading coefficients of A and B.

Define C=gcd(Ap,Bp)
*  If degree C = 0, return 1
NB this assumes monic polynomials;
the point is we have a trivial gcd

Define Known=p
Define Result=C

While Known ≤ 2LM do
p=nextPrime(A,B,p)
C=gcd(Ap,Bp)

If degree C < degree Result, goto *  all previous primes bad!
If degree C > degree Result, do nothing
If degree C = degree Result
Result=ChineseRemainder(C,Result,p,Known)
Known=Known*p

end while
```

Check Result divides A and B. If not, redo from start, avoiding all primes used so far.

Note that an early-exit strategy is possible- if the result after applying the chinese remainder theorem is unchanged, the odds are very good that you have found the true gcd. So the algorithm can be refined by testing for Result dividing A and B whenever this occurs, and halting the loop if successful.

References: CM30070 Computer Algebra, University of Bath- lecture notes, revision notes, and the lecturer's book of the same title. More in-depth information on the specifics of algorithms and bounds can be found in the book, although it is currently out of print. Details at http://www.bath.ac.uk/~masjhd/DSTeng.html

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