# Bézout's Identity

## Introduction to Bézout's identity

The GCD of two integers A, B is the largest positive integer that divides both of them leaving no remainder. Now because of Euclid's property that each integer N can be divided by another integer M as follows:

there exist pairs u,v such that we can write:

Since there is an infinite amount of those pairs, we'd like to find special ones. There are in fact exactly (A,B not being zero) two such pairs that satify

## Challenge

The goal of this challenge is to find the (ordered) pair of coefficients (u,v) that satify the above constraints and where u must be positive. This narrows down the output to a unique pair.

## Input

We may assume that the input is positive, also A will always be larger than B (A > B).

## Output

The output of our program/function must be the (ordered) pair specified in the challenge.

## Rules

One must not use built-in extended Euclidean algorithms (e.g. in Mathematica one is allowed to use GCD but not ExtendedGCD - which would fail for 5,3 anyways).

The answer may be a full program (taking input via STDIN or similar and output via STDOUT) or a function (returning the pair).

Beside the pair (u,v) there shall not be any output, trailing newlines or spaces are allowed. (brackets or commas are fine)

This is code golf, all standard loopholes are forbidden and the program with the lowest byte count wins.

## Examples

(A, B) -> (u, v)
(42, 12) -> (1, -3)
(4096, 84) -> (4, -195)
(5, 3) -> (2, -3)
(1155, 405) -> (20, -57)
(37377, 5204) -> (4365, -31351)
(7792, 7743) -> (7585, -7633)
(38884, 2737) -> (1707, -24251)
(6839, 746) -> (561, -5143)
(41908, 7228) -> (1104, -6401)
(27998, 6461) -> (3, -13)
(23780, 177) -> (20, -2687)
(11235813, 112358) -> (8643, -864301)


# MATL, 37 40 bytes

ZdXK2Gw/:1G*1GK/t_w2$:XI2G*!+K=2#fIb)  Uses release (9.3.1), which is earlier than this challenge. This is a brute force approach, so it may not work for large inputs. Try it online! The online compiler is based on a newer release, but produces the same results. ### Explanation Zd % implicitly input A and B. Compute their GCD. Call that C XK % copy C to clipboard K 2Gw/: % vector [1, 2, ..., B/C] 1G* % multiply that vector by A 1GK/t_w2$:    % vector [-A/C, -A/C+1 ..., A/C]
XI            % copy to clipboard I
2G*           % multiply that vector by B
!+            % all pairwise sums of elements from those vectors
K=2#f         % find row and column indices of sum that equals C
Ib)           % index second vector with column (indexing first vector with
% row is not needed, because that vector is of the form [1, 2, ...])


a#b=[(u,-v)|v<-[1..],u<-[1..v],gcd a b==u*a-v*b]!!0


Usage example: 27998 # 6461 -> (3,-13).

This is a brute force approach which finds all combinations of u and v that are valid solutions ordered by u and picks the first one. This takes some time to run for large |v|.

• I love the []!!0 idea=) – flawr Jan 19 '16 at 20:24

# Python 3, 101 106 bytes

Edit: Added some improvements and corrections suggested by Bruce_Forte.

An answer that uses the extended Euclidean algorithm. It's a bit clunky in places though, and I hope to golf it some more. I could convert to Python 2 to save a byte on integer division (//) but I'm not sure how Python 2's % modulus operator works with a negative second argument, as that is crucial for getting the output right.

def e(a,b):
r=b;x=a;s=z=0;t=y=1
while r:q=x/r;x,r=r,x%r;y,s=s,y-q*s;z,t=t,z-q*t
return y%(b/x),z%(-a/x)


Ungolfed:

def e(a, b):
r = b
x = a    # becomes gcd(a, b)
s = 0
y = 1    # the coefficient of a
t = 1
z = 0    # the coefficient of b
while r:
q = x / r
x, r = r, x % r
y, s = s, y - q * s
z, t = t, z - q * t
return y % (b / x), z % (-a / x) # modulus in this way so that y is positive and z is negative

• An anonymous user pointed out that the variable k in your ungolfed version's last line is undefined. – Jonathan Frech Dec 22 '18 at 18:40
• @JonathanFrech Ah, thank you! – Sherlock9 Jan 7 at 14:10

# Mathematica, 80 bytes

f@l_:=Mod@@NestWhile[{Last@#,{1,-Quotient@@(#.l)}.#}&,{{1,0},{0,1}},Last@#.l>0&]


Explanation:

Extended Euclidean algorithm is used here, in a Nest style. The method that the coefficients are stored in arrays makes it possible to use Dot.

Another possible representation is simply using symbolic expression, like u a - v b with {a->19, b->17}. Such representation makes use of the feature of Mathematica and is interesting, but it is much longer in bytes.

Test cases:

f[{5, 3}]              (* {2, -3} *)
f[{42, 12}]            (* {1, -3} *)
f[{11235813, 112358}]  (* {8643, -864301} *)


# Ruby, 83 bytes

I think there are a few ways to fine-tune and golf this solution, but I like it so far. Maybe I'll try an extended Euclidean algorithm solution next.

->x,y{a=b=0;y.downto(0).map{|u|(-x..0).map{|v|a,b=u,v if u*x+v*y==x.gcd(y)}};p a,b}


How it works

This code starts with a loop of u from y down to 0, with an inner loop of v from -x to 0, inside which we check every u and v if u*x+v*y == gcd(x, y). Since there could be multiple matches along the way (this uses a very exhaustive search), we start far from 0 so that when we get the last of the multiple matches, it's the one where |u| and |v| are closest to 0.

def bezout(x,y)
a=b=0
y.downto(0).each do |u|
(-x..0).each do |v|
if u*x + v*y == x.gcd(y)
a,b=u,v
end
end
end
p a,b
end

• @Bruce_Forte Darn. IRB ran out of memory for that test case. I'll write up an extended Euclidean algorithm solution as soon as I can. – Sherlock9 Jan 19 '16 at 18:05