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@@ -11,15 +11,15 @@ $$
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#### Euclidean Algorithm
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- The euclidean algorithm calculates the greatest common divisor of two numbers $gcd(r_0, r_1)$
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- This is the largest number that divides both $r_0$ and $r_1$
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- The Euclidean algorithm calculates the greatest common divisor of two numbers $gcd(r_0, r_1)$
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- This is the largest number that divides both $r_0$ and $r_1$
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- If $gcd(x,y)=1$ then $x$ and $y$ are **coprime** (sometimes called relatively prime)
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- The Euclidean algorithm is based around the fact:
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- $gcd(r_0, r_1) = gcd(r_1, r_0 - r_1)$
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- $gcd(r_0, r_1) = gcd(r_1, r_0 - r_1)$
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- Computing $(x-y)\cdot gcd(r_0, r_1)$ is easier as its a smaller number
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- Computing $(x-y)\cdot gcd(r_0, r_1)$ is easier as it's a smaller number
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- Doing this repeatedly is slow, we can use $gcd(r_0,r_1) = gcd(r_1, r_0\space mod \space r_1)$
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@@ -37,16 +37,16 @@ $r_0=q\cdot r_1 + r_2 \\57=4\cdot 12 + 9\\ r_1=q\cdot r_2 + r_3 \\ 12=1\cdot 9 +
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#### Bezout’s Identity
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#### Bézout’s Identity
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- Bezout’s identity tells us that the greatest common divisor of two numbers can be expressed as the sum of multiples of these numbers
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- Bézout’s identity tells us that the greatest common divisor of two numbers can be expressed as the sum of multiples of these numbers
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- $gcd(r_0,r_1) = s\cdot r_0 + t\cdot r_1$
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- e.g. $gcd(99,20)=-1\cdot 99+5\cdot 20=1$
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- $gcd(141,50)=11\cdot 141+-31\cdot 50=1$
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- e.g. $gcd(99,20)=-1\cdot 99+5\cdot 20=1$
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- $gcd(141,50)=11\cdot 141+-31\cdot 50=1$
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##### Extended Euclidean Algorithm
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- The extended euclidean algorithm calculates the $gcd(r_0,r_1)$ as normal, and in addition calculates $s$ and $t$.
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- The extended Euclidean algorithm calculates the $gcd(r_0,r_1)$ as normal, and in addition calculates $s$ and $t$.
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| Euclidean Algorithm | Extended Euclidean Algorithm |
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| ---------------------------------- | ------------------------------------------------------------ |
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@@ -81,4 +81,3 @@ $$
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$$
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Where $t$ is our multiplicative inverse
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