Given that , find the number of natural numbers that satisfies:
Notation: denotes the floor function.
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Consider k 3 ≤ n < ( k + 1 ) 3 where k is a positive integer
So, ⌊ 3 n ⌋ = k
Therefore every integer within this range can be expressed as n = k 3 + r where 0 ≤ r ≤ 3 k 2 + 3 k
When ⌊ 3 n ⌋ ∣ n that is k ∣ k 3 + r so, k ∣ r .Now r will be 0 , k , 2 k , 3 k , . . . . . , 3 k 2 + 3 k . So, total number of such r is 3 ( k + 1 ) + 1 = 3 k + 4
As 1 2 3 < 2 0 1 7 < 1 3 3
So, total solutions upto 1 2 3 − 1 is k = 1 ∑ 1 1 3 k + 4 = 2 4 2
Again the last part of the set is 1 2 3 , 1 2 3 + 1 2 , . . . . . 2 0 1 6 there are 2 5 more solutions.
So, total number of solutions are 2 4 2 + 2 5 = 2 6 7