Divisibility #2

Given that n < 2017 n<2017 , find the number of natural numbers n n that satisfies:

n 3 n \large \lfloor \sqrt[3]{n} \rfloor \mid n

Notation: \lfloor\cdot \rfloor denotes the floor function.


The answer is 267.

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1 solution

Kushal Bose
Jun 16, 2017

Consider k 3 n < ( k + 1 ) 3 k^3 \leq n < (k+1)^3 where k k is a positive integer

So, n 3 = k \lfloor \sqrt[3]{n} \rfloor =k

Therefore every integer within this range can be expressed as n = k 3 + r n=k^3+r where 0 r 3 k 2 + 3 k 0 \leq r \leq 3k^2+3k

When n 3 n \lfloor \sqrt[3]{n} \rfloor | n that is k k 3 + r k | k^3+r so, k r k|r .Now r r will be 0 , k , 2 k , 3 k , . . . . . , 3 k 2 + 3 k 0,k,2k,3k,.....,3k^2+3k . So, total number of such r r is 3 ( k + 1 ) + 1 = 3 k + 4 3(k+1)+1=3k+4

As 1 2 3 < 2017 < 1 3 3 12^3 < 2017 <13^3

So, total solutions upto 1 2 3 1 12^3-1 is k = 1 11 3 k + 4 = 242 \displaystyle \sum_{k=1}^{11} 3k+4=242

Again the last part of the set is 1 2 3 , 1 2 3 + 12 , . . . . . 2016 12^3,12^3+12,.....2016 there are 25 25 more solutions.

So, total number of solutions are 242 + 25 = 267 242+25=267

Nice! I was actually going to post a solution myself, but your solution is almost exactly same with mine! xD

Boi (보이) - 3 years, 12 months ago

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Thanks Bro....

Kushal Bose - 3 years, 12 months ago

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