GCD Raised too!

If a a and b b are positive integers such that gcd ( a , b ) = 13 \gcd(a,b)=13 , then find the sum of all distinct values of gcd ( a 3 , b ) \gcd(a^3,b) .

Notation: gcd ( , ) \gcd(\, \cdot \, , \cdot) denotes the greatest common divisor function.


The answer is 2379.

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2 solutions

Let's say for a positive integer x x and a non-negative integer c c and prime p p , " p p -adic order/valuation of x x is c c ", or write " v p ( x ) = c v_p(x)=c ", to mean p c x p^c \mid x but p c + 1 ∤ x p^{c+1}\not \mid x ; that is, p c p^c is the exact power of p p that divides x x ; (This is, in fact, an standard terminology).

Now, as 13 13 is a prime and g c d ( a , b ) = 13 gcd(a,b)=13 , then 13 13 -adic order of both a a and b b is at least 1 1 ; and 13 13 -adic order is 1 1 for at least one of a a and b b . Here arise 3 3 cases.

Case-I : v 13 ( a ) 1 v_{13}(a)\geq1 and v 13 ( b ) = 1 v_{13}(b)=1

Then g c d ( a 3 , b ) = 13 gcd(a^3,b)=13 [As g c d ( a , b ) = 13 gcd(a,b)=13 ]

Case-II : v 13 ( a ) = 1 v_{13}(a)=1 and v 13 ( b ) = 2 v_{13}(b)=2

Now, v 13 ( a 3 ) = 3 v_{13}(a^3)=3 . Then g c d ( a 3 , b ) = 1 3 2 gcd(a^3,b)=13^2 [As g c d ( a , b ) = 13 gcd(a,b)=13 ]

Case-III : v 13 ( a ) = 1 v_{13}(a)=1 and v 13 ( b ) 3 v_{13}(b)\geq3

Again, v 13 ( a 3 ) = 3 v_{13}(a^3)=3 . Then g c d ( a 3 , b ) = 1 3 3 gcd(a^3,b)=13^3 [As g c d ( a , b ) = 13 gcd(a,b)=13 ]

So, ( 13 + 1 3 2 + 1 3 3 ) = 2379 (13+13^2+13^3)=\boxed{2379} is the answer.

How about the more general case of gcd ( a , b ) = n \gcd (a,b) = n and we're looking for values of gcd ( a i , b j ) \gcd ( a^i, b^j ) ?

Calvin Lin Staff - 4 years, 5 months ago

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We can consider each prime factor of n n (along with it's exponent in n n ) separately.

Then multiplying the sum for each prime p p , I'm expecting a formula for sum of distinct values of n n that looks like the SUM OF DIVISOR function.

Muhammad Rasel Parvej - 4 years, 5 months ago
Prince Loomba
Jan 1, 2017

g c d ( a , b ) = 13 gcd (a,b)=13 implies a a has 13 13 as a factor and b b has too. So let a = p × 1 3 x a=p×13^{x} and b = q × 1 3 y b=q×13^{y} where p p and q q are coprime, Let x = 1 x=1 . So y y can be any natural number.

Now for g c d ( a 3 , b ) gcd (a^{3},b) It can be 13 , 1 3 2 , 1 3 3 13, 13^{2},13^{3} and not more as x = 1 x=1 .

Let x > 1 x>1 . So y = 1 y=1 because g c d ( a , b ) gcd(a,b) is 13 13 . So whatever shall be x x , g c d ( a 3 , b ) gcd (a^{3},b) will be 13 13 .

Thus different values= 13 , 1 3 2 , 1 3 3 13,13^2,13^3 .

Sum= 2379 \boxed {2379}

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