If a and b are positive integers such that g cd ( a , b ) = 1 3 , then find the sum of all distinct values of g cd ( a 3 , b ) .
Notation: g cd ( ⋅ , ⋅ ) denotes the greatest common divisor function.
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How about the more general case of g cd ( a , b ) = n and we're looking for values of g cd ( a i , b j ) ?
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We can consider each prime factor of n (along with it's exponent in n ) separately.
Then multiplying the sum for each prime p , I'm expecting a formula for sum of distinct values of n that looks like the SUM OF DIVISOR function.
g c d ( a , b ) = 1 3 implies a has 1 3 as a factor and b has too. So let a = p × 1 3 x and b = q × 1 3 y where p and q are coprime, Let x = 1 . So y can be any natural number.
Now for g c d ( a 3 , b ) It can be 1 3 , 1 3 2 , 1 3 3 and not more as x = 1 .
Let x > 1 . So y = 1 because g c d ( a , b ) is 1 3 . So whatever shall be x , g c d ( a 3 , b ) will be 1 3 .
Thus different values= 1 3 , 1 3 2 , 1 3 3 .
Sum= 2 3 7 9
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Let's say for a positive integer x and a non-negative integer c and prime p , " p -adic order/valuation of x is c ", or write " v p ( x ) = c ", to mean p c ∣ x but p c + 1 ∣ x ; that is, p c is the exact power of p that divides x ; (This is, in fact, an standard terminology).
Now, as 1 3 is a prime and g c d ( a , b ) = 1 3 , then 1 3 -adic order of both a and b is at least 1 ; and 1 3 -adic order is 1 for at least one of a and b . Here arise 3 cases.
Case-I : v 1 3 ( a ) ≥ 1 and v 1 3 ( b ) = 1
Case-II : v 1 3 ( a ) = 1 and v 1 3 ( b ) = 2
Case-III : v 1 3 ( a ) = 1 and v 1 3 ( b ) ≥ 3
So, ( 1 3 + 1 3 2 + 1 3 3 ) = 2 3 7 9 is the answer.