How many discrete numbers are there in the following set?
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Imagine the curve y ( x ) = x 2 0 1 8 on real line. The derivative of the curve is y ′ = − x 2 2 0 1 8 . We are gonna find the point, at which the derivative gets lower than one, in magnitude, by putting the derivative equal to minus one. That would be at x = 2 0 1 8 ≈ 4 4 . 9 . When 1 ≤ i ≤ 4 4 , ⌊ i 2 0 1 8 ⌋ takes 4 4 different values, because the absolute value of the derivative is higher than 1 and i + 1 2 0 1 8 < t − 1 , for t ∈ R , if i 2 0 1 8 = t . For 4 5 ≤ i ≤ 2 0 1 8 , the derivative would be less than one, in magnitude, therefore, all the values from ⌊ 4 5 2 0 1 8 ⌋ = 4 4 to 1 , would be covered and there are 4 4 of them. So, there would be a total of 4 4 + 4 4 = 8 8 values that are produced.
Hey, I cannot upvote my own solution!
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If you think about the floor function, repeated values start when
⌊ i 2 0 1 8 ⌋ = ⌊ i + 1 2 0 1 8 ⌋
So, let's consider the bound
1 ≥ i 2 0 1 8 − i + 1 2 0 1 8
This gives us an idea of where ⌊ i 2 0 1 8 ⌋ might start having repeated values.
( i + 1 ) ( i ) − 2 0 1 8 ≥ 0 , ( note: i ∈ Z )
i ≥ 4 5
For 1 ≤ i ≤ 4 4 , results are 4 4 different discrete numbers, because consecutive results differ by more than 1.
For 4 5 ≤ i ≤ 2 0 1 8 , consecutive results will either differ by 1 or 0 (repeat), so unique results will decrease by 1 from ⌊ 4 5 2 0 1 8 ⌋ = 4 4 to ⌊ 2 0 1 8 2 0 1 8 ⌋ = 1 . So, other 4 4 discrete numbers are in the set.
4 4 + 4 4 = 8 8