Add GCD Properties

[LuchoBazz]

Properties of GCD (Greatest Common Divisor)

• Blueprint of Numbers: The GCD of a set of numbers can be thought of as a blueprint of those numbers. If you keep adding the GCD, you can make all numbers that belong to that set.

• Common Divisors: Every common divisor of and is a divisor of .

• Linear Combination: , where both and are non-zero, can also be defined as the smallest positive integer which can be expressed as a linear combination of and in the form

  

  where both and are integers.

• GCD with Zero:

  

  since any number is a divisor of 0, and the greatest divisor of is .

• Division Property: If divides and , then

  

• Scaling Property: If is a non-negative integer, then

  

  It also follows from this property that if , then

  

• Translation Property: If is any integer,

  

• Euclidean Algorithm: The GCD can be found using the Euclidean algorithm:

  

• Common Divisor Scaling: If is a positive common divisor of and , then

  

• Multiplicative Function: The GCD is a multiplicative function. That is, if and are coprime,

  

  In particular, recalling that GCD is a positive integer-valued function, we obtain that

  

  If the GCD is one, then they need not be coprime to distribute the GCD; moreover, each GCD individually should also be 1.

• Commutative Property: The GCD is a commutative function:

  

• Associative Property: The GCD is an associative function:

  

  Thus,

  

  can be used to denote the GCD of multiple arguments.

• Relation with LCM: is closely related to the least common multiple : we have

  

• Distributivity Versions: The following versions of distributivity hold true:

  

  

• Prime Factorization: If we have the unique prime factorizations of and , where and , then the GCD of and is:

  

• Cartesian Coordinate System Interpretation: In a Cartesian coordinate system, can be interpreted as the number of segments between points with integral coordinates on the straight line segment joining the points and .

• Euclidean Algorithm in Base : For non-negative integers and , where and are not both zero, provable by considering the Euclidean algorithm in base , it simply states that:

  

  If you want an informal proof, think of numbers in base 2. We are calculating GCDs of numbers which contain all continuous 1s in their binary representations. For example: 001111 and 000011; their GCD can be the greatest common length, which in this case is 2. Thus, the GCD becomes 000011. Think of numbers in terms of length; maybe you get the idea.

• Euler's Totient Function Identity: An identity involving Euler's totient function:

  

  where are all common divisors of and .

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```tex # Properties of GCD (Greatest Common Divisor) - **Blueprint of Numbers:** The GCD of a set of numbers can be thought of as a blueprint of those numbers. If you keep adding the GCD, you can make all numbers that belong to that set. - **Common Divisors:** Every common divisor of $$\(a\)$$ and $$\(b\)$$ is a divisor of $$\(\gcd(a,b)\)$$. - **Linear Combination:** $$\(\gcd(a,b)\)$$, where both $$\(a\)$$ and $$\(b\)$$ are non-zero, can also be defined as the smallest positive integer $$\(d\)$$ which can be expressed as a linear combination of $$\(a\)$$ and $$\(b\)$$ in the form $$ d = a \cdot p + b \cdot q $$ where both $$\(p\)$$ and $$\(q\)$$ are integers. - **GCD with Zero:** $$ \gcd(a, 0) = |a|, \text{ for } a \neq 0 $$ since any number is a divisor of 0, and the greatest divisor of $$\(a\)$$ is $$\(|a|\)$$. - **Division Property:** If $$\(a\)$$ divides $$\(b \cdot c\)$$ and $$\(\gcd(a,b) = d\)$$, then $$ \frac{a}{d} \text{ divides } c $$ - **Scaling Property:** If $$\(m\)$$ is a non-negative integer, then $$ \gcd(m \cdot a, m \cdot b) = m \cdot \gcd(a, b) $$ It also follows from this property that if $$\(\gcd(a,b) = g\)$$, then $$ \frac{a}{g} \text{ and } \frac{b}{g} \text{ should be coprime.} $$ - **Translation Property:** If $$\(m\)$$ is any integer, $$ \gcd(a,b) = \gcd(a + m \cdot b, b) $$ - **Euclidean Algorithm:** The GCD can be found using the Euclidean algorithm: $$ \gcd(a, b) = \gcd(b, a \mod b) $$ - **Common Divisor Scaling:** If $$\(m\)$$ is a positive common divisor of $$\(a\)$$ and $$\(b\)$$, then $$ \gcd\left(\frac{a}{m}, \frac{b}{m}\right) = \frac{\gcd(a, b)}{m} $$ - **Multiplicative Function:** The GCD is a multiplicative function. That is, if $$\(a_1\)$$ and $$\(a_2\)$$ are coprime, $$ \gcd(a_1 \cdot a_2, b) = \gcd(a_1, b) \cdot \gcd(a_2, b) $$ In particular, recalling that GCD is a positive integer-valued function, we obtain that $$ \gcd(a, b \cdot c) = 1 \text{ if and only if } \gcd(a, b) = 1 \text{ and } \gcd(a, c) = 1. $$ If the GCD is one, then they need not be coprime to distribute the GCD; moreover, each GCD individually should also be 1. - **Commutative Property:** The GCD is a commutative function: $$ \gcd(a, b) = \gcd(b, a) $$ - **Associative Property:** The GCD is an associative function: $$ \gcd(a, \gcd(b, c)) = \gcd(\gcd(a, b), c) $$ Thus, $$ \gcd(a, b, c, \ldots) $$ can be used to denote the GCD of multiple arguments. - **Relation with LCM:** $$\(\gcd(a, b)\)$$ is closely related to the least common multiple $$\(\operatorname{lcm}(a, b)\)$$: we have $$ \gcd(a, b) \cdot \operatorname{lcm}(a, b) = |a \cdot b| $$ - **Distributivity Versions:** The following versions of distributivity hold true: $$ \gcd(a, \operatorname{lcm}(b, c)) = \operatorname{lcm}(\gcd(a, b), \gcd(a, c)) $$ $$ \operatorname{lcm}(a, \gcd(b, c)) = \gcd(\operatorname{lcm}(a, b), \operatorname{lcm}(a, c)) $$ - **Prime Factorization:** If we have the unique prime factorizations of $$\(a = p_1^{e_1} p_2^{e_2} \cdots p_m^{e_m}\)$$ and $$\(b = p_1^{f_1} p_2^{f_2} \cdots p_m^{f_m}\)$$, where $$\(e_i \geq 0\)$$ and $$\(f_i \geq 0\)$$, then the GCD of $$\(a\)$$ and $$\(b\)$$ is: $$ \gcd(a,b) = p_1^{\min(e_1,f_1)} p_2^{\min(e_2,f_2)} \cdots p_m^{\min(e_m,f_m)} $$ - **Cartesian Coordinate System Interpretation:** In a Cartesian coordinate system, $$\(\gcd(a, b)\)$$ can be interpreted as the number of segments between points with integral coordinates on the straight line segment joining the points $$\((0, 0)\)$$ and $$\((a, b)\)$$. - **Euclidean Algorithm in Base $$\(n\)$$:** For non-negative integers $$\(a\)$$ and $$\(b\)$$, where $$\(a\)$$ and $$\(b\)$$ are not both zero, provable by considering the Euclidean algorithm in base $$\(n\)$$, it simply states that: $$ \gcd(n^a - 1, n^b - 1) = n^{\gcd(a, b)} - 1 $$ If you want an informal proof, think of numbers in base 2. We are calculating GCDs of numbers which contain all continuous 1s in their binary representations. For example: 001111 and 000011; their GCD can be the greatest common length, which in this case is 2. Thus, the GCD becomes 000011. Think of numbers in terms of length; maybe you get the idea. - **Euler's Totient Function Identity:** An identity involving Euler's totient function: $$ \gcd(a,b) = \sum \phi(k) $$ where $$\(k\)$$ are all common divisors of $$\(a\)$$ and $$\(b\)$$. ```

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• gcd(a, b) = gcd(a - b, b)