2016 is absolutely awesome 1

Algebra Level 5

2016 2016 k = k ; 2016 2016 k = k \left\lfloor\dfrac{2016}{\left\lfloor\dfrac{2016}{k}\right\rfloor}\right\rfloor=k\quad;\quad \left\lceil\dfrac{2016}{\left\lceil\dfrac{2016}{k}\right\rceil}\right\rceil=k

An integer 1 k 2016 1\le k\le 2016 is called 2016-awesome if it satisfies the equations above.

Find the number of 2016-awesome numbers.


This is a part of the Set .


The answer is 67.

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1 solution

For the following derivations, we assume that k doesn't divide 2016.

Let j = 2016 k j=\left\lfloor \frac { 2016 }{ k } \right\rfloor . From the first equation we have that: 2016 k + 1 < j 2016 k \frac { 2016 }{ k+1 } <j\le \frac { 2016 }{ k } Provided that there is an integer value that j j can take in ( 2016 k + 1 , 2016 k ] \left( \frac { 2016 }{ k+1 } ,\frac { 2016 }{ k } \right] then eq. 1 will be met. This imples that the first equation must be met if the length of the interval is greater or equal to 1. From this we get:

2016 k 2016 k + 1 = 2016 k ( k + 1 ) 1 \frac { 2016 }{ k } -\frac { 2016 }{ k+1 } =\frac { 2016 }{ k(k+1) } \ge 1 0 k 2 + k 2016 0\ge { k }^{ 2 }+k-2016 1 8065 2 k 1 + 8065 2 \frac { -1-\sqrt { 8065 } }{ 2 } \le k\le \frac { -1+\sqrt { 8065 } }{ 2 } k 44 k \le 44

Similarly, letting i = 2016 k i=\left\lceil \frac { 2016 }{ k } \right\rceil . From the second equation we have that: 2016 k i < 2016 k 1 \frac { 2016 }{ k } \le i<\frac { 2016 }{ k-1 }

By similar reasoning to the previous derivation, we get that:

2016 k 1 2016 k = 2016 k ( k 1 ) 1 \frac { 2016 }{ k-1 } -\frac { 2016 }{ k } =\frac { 2016 }{ k(k-1) } \ge 1 0 k 2 k 2016 0\ge { k }^{ 2 }-k-2016 1 8065 2 k 1 + 8065 2 \frac { 1-\sqrt { 8065 } }{ 2 } \le k\le \frac { 1+\sqrt { 8065 } }{ 2 } k 45 k \le 45

We no know that all k 44 k \le 44 satisfy both equations.

Since k doesn't divide 2016, j + 1 = i j+1=i .

If both equations are met, then necessarily:

2016 j = 2016 j + 1 \left\lfloor \frac { 2016 }{ j } \right\rfloor =\left\lceil \frac { 2016 }{ j+1 } \right\rceil 2016 j = 2016 j + 1 + α \frac { 2016 }{ j } =\frac { 2016 }{ j+1 } +\alpha

Where 0 < α < 2 0<\alpha <2 . This in turn implies:

α = 2016 j ( j + 1 ) \alpha =\frac { 2016 }{ j(j+1) } j = 1 + 1 + 8064 / α 2 j=\frac { -1+\sqrt { 1+8064/\alpha } }{ 2 } j > 1 + 1 + 8064 / 2 2 j> \frac { -1+\sqrt { 1+8064/2 } }{ 2 } j 32 j\ge 32 k 61 k\le 61

Therefore, if k doesn't divide 2016, it can only satisfy both equations if it is less than or equal to 61. We now check the interval 44 < k 61 44<k\le61 . In it 45, 46, 48, 50, 51, 54 and 56 satisfy both equations. So far, we found 44 + 7 = 51 44+7=51 cases where both equations are met. The remaining numbers that satisfy the equations are the factors of 2016 that we haven't already counted. These are: 63, 72, 84, 96, 112, 126, 144, 168, 224, 252, 288, 336, 504, 672, 1008 and 2016.

For numbers greater that 2016, eq. one gives 2016 0 k \left\lfloor \frac { 2016 }{ 0 } \right\rfloor \neq k , so there are no other solutions.

So, in total there are 67 2016-awesome integers.

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