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Friday, January 26, 2024

2024/005) For m > 1 show that if 2^{2m+1} > n^2 then 2^{2m+1} \ge n^2+7

Because m > 1 2m + 1\ge 4  So    2^{2m+1} is divisible by 16.

  2^{2m+1} is double of (2^m)^2 and is not a square.

Now we consider 2 cases

Let us consider when n is odd 

Then n= (2k+1)

So n^2 = (2k+1)^2 = 4k^2 + 4k + 1 = 4k(k+1)+1

Ss k or k+1 is even we n^2 \equiv 1 \pmod 8 

So 2^{2m+1} is divisible by 8 and minimum number above n^2 which is divisible by 8 is n^2 + 7

So we have proved for the case n is odd.

Let us consider when n is even

If  n^2 is divisible by 8 then n^2 is divisible by 16 and as 2^{2m+1} is divisible by 16 and n^2 is divisible by 16 

So 2^{2m+1}-n^2 = 16k for some positive k

So 2^{2m+1}-n^2 \ge16

Hence  2^{2m+1}-n^2 \gt 7 or 2^{2m+1} \gt n^2+ 7

If n^2 is even and is not divisible by 8 then n is of the form 4k+2

n^2= 16k^2 + 16k + 4

Or n^2 \equiv 4 \pmod {16} 

 As 2^{2m+1} = 0 \pmod {16}

So 2^{2m+1} \ge n^2 + 12

Hence  2^{2m+1} \gt n^2 + 7

 we have proved for all 3 cases hence done

 


 

  

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