Once there was a 9

Determine all positive integer values of n n such that

( n 0 ) + ( n 1 ) + ( n 2 ) + ( n 3 ) 2 2008 { n \choose 0 } + { n \choose 1 } + { n \choose 2 } + { n \choose 3 } \ \bigg| \ 2 ^ { 2008 }

What is the sum of all these values?


The answer is 36.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Patrick Corn
Nov 27, 2017

The sum of the first four binomial coefficients equals ( n + 1 ) ( n 2 n + 6 ) 6 . \frac{(n+1)(n^2-n+6)}6. So ( n + 1 ) ( n 2 n + 6 ) 3 2 2009 . (n+1)(n^2-n+6) | 3 \cdot 2^{2009}. There are two cases.

Case 1: n + 1 = 2 a n+1 = 2^a for some a . a. Then n 2 n + 6 = 2 2 a 3 2 a + 8. n^2-n+6 = 2^{2a} - 3 \cdot 2^a + 8. This is either a power of 2 2 or 3 3 times a power of 2. 2.

Let x = 2 a . x = 2^a. Then I claim that for x > 8 , x > 8, 3 4 x 2 < x 2 3 x + 8 < 3 2 x 2 . \frac34 x^2 < x^2-3x+8 < \frac32 x^2. This is an easy algebra exercise, left to the reader. But this means that 2 2 a 3 2 a + 8 2^{2a}-3\cdot 2^a + 8 is strictly between 3 2 2 a 2 3 \cdot 2^{2a-2} and 3 2 2 a 1 , 3 \cdot 2^{2a-1}, so it can't be 3 3 times a power of 2 2 if a > 3. a > 3.

Similarly, 1 2 x 2 < x 2 3 x + 8 < x 2 \frac12 x^2 < x^2 -3x+8 < x^2 for x > 8 , x > 8, so 2 2 a 3 2 a + 8 2^{2a}-3\cdot 2^a + 8 is strictly between 2 2 a 1 2^{2a-1} and 2 2 a , 2^{2a}, so it can't be a power of 2 2 if a > 3. a > 3.

That reduces the possible values of a a to a = 0 , 1 , 2 , 3. a = 0,1,2,3. Checking directly, we see that a = 1 , 2 , 3 a = 1,2,3 give the solutions n = 1 , 3 , 7 n=1,3,7 respectively.

Case 2: n + 1 = 3 2 a n+1 = 3 \cdot 2^a for some a . a. Then n 2 n + 6 = 9 2 2 a 9 2 a + 8. n^2-n+6 = 9 \cdot 2^{2a} - 9 \cdot 2^a + 8. This is supposed to be a power of 2. 2. Again, let x = 2 a . x = 2^a. This time I claim that for x > 8 , x > 8, 8 x 2 < 9 x 2 9 x + 8 < 16 x 2 . 8x^2 < 9x^2-9x+8 < 16x^2. So 9 2 2 a 9 2 a + 8 9 \cdot 2^{2a} - 9 \cdot 2^a + 8 is strictly between 2 2 a + 3 2^{2a+3} and 2 2 a + 4 , 2^{2a+4}, so can't be a power of 2 2 if a > 3. a > 3. As before, we have only the four cases a = 0 , 1 , 2 , 3 a=0,1,2,3 to check. Here a = 0 , 3 a=0,3 lead to the solutions n = 2 , 23 n=2,23 respectively.

So the answer is 1 + 3 + 7 + 2 + 23 = 36 . 1+3+7+2+23 = \fbox{36}.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...