Logarithmic Arithmetic Derivative

d d x n n 0 ? \Large \frac{\frac{d}{dx}n}{n} \neq 0?

In calculus, when you take the derivative of a constant you get zero as an answer. In number theory, there is something called the arithmetic derivative which allows you to differentiate a number and get a nonzero answer. The arithmetic derivative works as follows.

Where n n' denotes the arithmetic derivative of n n :

p = 1 p' = 1 for all primes p p

( a b ) = a b + a b (ab)'=a'b+ab'

0 = 1 = 0 0'=1'=0

For example, 6 = ( 2 × 3 ) = ( 2 ) ( 3 ) + ( 2 ) ( 3 ) = ( 1 ) ( 3 ) + ( 2 ) ( 1 ) = 5 6'=(2\times3)'=(2')(3)+(2)(3')=(1)(3)+(2)(1)=5

Let us define the Logarithmic Arithmetic Derivative , denoted by L ( n ) L(n) , as n n \frac{n'}{n} . For how many ordered pairs ( a , b ) (a,b) , where a a and b b are distinct positive integers less than 100 100 , does L ( a ) = L ( b ) L(a)=L(b) ?

This is a member of a set of problems on the Arithmetic Derivative .


The answer is 6.

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1 solution

Kazem Sepehrinia
Jul 17, 2015

a = b a=b is a trivial solutions. For a b a \neq b L ( a ) = L ( b ) a a = b b a b b a = 0 a b b a b 2 = 0 ( a b ) = 0 L(a)=L(b) \\ \frac{a'}{a}=\frac{b'}{b} \\ a'b-b'a=0 \\ \frac{a'b-b'a}{b^2}=0 \\ \left(\frac{a}{b}\right)'=0 From solution to this problem , if a b = i = 1 k p i x i \frac{a}{b}=\prod_{i=1}^{k} p_i^{x_i} then ( a b ) = a b i = 1 k x i p i \left(\frac{a}{b}\right)'=\frac{a}{b} \sum_{i=1}^{k} \frac{x_i}{p_i} Now for this problem ( a b ) = a b i = 1 k x i p i = 0 i = 1 k x i p i = 0 x 1 p 2 p 3 . . . p k + x 2 p 1 p 3 . . . p k + . . . + x k p 1 p 2 . . . p k = 0 \left(\frac{a}{b}\right)'=\frac{a}{b} \sum_{i=1}^{k} \frac{x_i}{p_i}=0 \\ \sum_{i=1}^{k} \frac{x_i}{p_i}=0 \\ x_1 p_2 p_3 ... p_k+x_2 p_1 p_3 ... p_k+ ... + x_k p_1 p_2 ... p_k=0 \ \ \ \ \star In the same manner of solution to above mentioned problem one can conclude that x i = c i p i x_i=c_i p_i and equation \star becomes c 1 + c 2 + . . . + c k = 0 c_1+c_2+...+c_k=0 . Therefore the solutions to the equation ( a b ) = 0 \left(\frac{a}{b}\right)'=0 \ \ \ \dagger are in the form a b = i = 1 k ( p i p i ) c i \frac{a}{b}=\prod_{i=1}^{k} (p_i^{p_i})^{c_i} where c i = 0 \sum c_i=0 .

For this problem since a a and b b are less than 100 100 only possible values for a b \frac{a}{b} are a b = 2 2 3 3 , 3 3 2 2 \frac{a}{b}=\frac{2^2}{3^3}, \frac{3^3}{2^2} Now if a b \frac{a}{b} is a solution to the equation \dagger then m a m b \frac{ma}{mb} is a solution too. So the solutions are ( 2 2 , 3 3 ) , ( 2 3 , 2 × 3 3 ) , ( 3 × 2 2 , 3 4 ) , ( 3 3 , 2 2 ) , ( 2 × 3 3 , 2 3 ) , ( 3 4 , 3 × 2 2 ) (2^2, 3^3), (2^3, 2\times 3^3), (3\times 2^2, 3^4), (3^3, 2^2), (2\times 3^3, 2^3), (3^4, 3\times 2^2)

You have a typo and list (24,81) when it should be (12,81). Also, the question should make it clearer that (4,27) and (27,4) are both counted, but maybe that's just my sloppiness.

Theodore Anthiros - 5 years, 10 months ago

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