The god of Number Theory

How many discrete numbers are there in the following set?

88 89 87 86

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2 solutions

If you think about the floor function, repeated values start when

2018 i = 2018 i + 1 \lfloor \frac{2018}{i} \rfloor = \lfloor \frac{2018}{i+1} \rfloor

So, let's consider the bound

1 2018 i 2018 i + 1 1 \geq \frac{2018}{i} - \frac{2018}{i+1}

This gives us an idea of where 2018 i \lfloor \frac{2018}{i} \rfloor might start having repeated values.

( i + 1 ) ( i ) 2018 0 (i+1)(i) - 2018 \geq 0 , ( note: i Z i \in \mathbb{Z} )

i 45 i \geq 45

For 1 i 44 1 \leq i \leq 44 , results are 44 44 different discrete numbers, because consecutive results differ by more than 1.

For 45 i 2018 45 \leq i \leq 2018 , consecutive results will either differ by 1 or 0 (repeat), so unique results will decrease by 1 from 2018 45 = 44 \lfloor \frac{2018}{45} \rfloor = 44 to 2018 2018 = 1 \lfloor \frac{2018}{2018} \rfloor = 1 . So, other 44 44 discrete numbers are in the set.

44 + 44 = 88 44+44 = 88

Imagine the curve y ( x ) = 2018 x y(x)=\frac{2018}{x} on real line. The derivative of the curve is y = 2018 x 2 y'= -\frac{2018}{x^2} . 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 = 2018 44.9 x=\sqrt{2018}\approx 44.9 . When 1 i 44 1\leq i \leq 44 , 2018 i \lfloor \frac{2018}{i}\rfloor takes 44 44 different values, because the absolute value of the derivative is higher than 1 1 and 2018 i + 1 < t 1 \frac{2018}{i+1} < t-1 , for t R t \in \mathbb{R} , if 2018 i = t \frac{2018}{i} =t . For 45 i 2018 45\leq i \leq 2018 , the derivative would be less than one, in magnitude, therefore, all the values from 2018 45 = 44 \lfloor \frac{2018}{45}\rfloor=44 to 1 1 , would be covered and there are 44 44 of them. So, there would be a total of 44 + 44 = 88 44+44=88 values that are produced.

Hey, I cannot upvote my own solution!

A Former Brilliant Member - 2 years, 8 months ago

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