Mod 8

How many different integer values x x such that 0 x 7 0 \le x \le 7 and there is some integer n n such that n 2 x (mod 8) n^2 \equiv x \text{ (mod 8)} ?

6 3 2 8

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2 solutions

Josh Speckman
May 13, 2014

Claim: All perfect squares are equivalent to 0 0 , 1 1 , or 4 4 (mod 8) \text{(mod 8)} . Proof: If x x is even, then x 2 x^2 is divisible by 4 4 , meaning it is equivalent to 0 0 or 4 4 (mod 8) \text{(mod 8)} . Since 8 2 0 8^2 \equiv 0 (mod 8) \text{(mod 8)} and 2 2 4 2^2 \equiv 4 (mod 8) \text{(mod 8)} , both cases are possible.

If x x is odd, then x x can be written in the form 2 y + 1 2y+1 for some integer y y . Squaring this yields 4 y 2 + 4 y + 1 4y^2+4y+1 . We factor this as 4 y ( y + 1 ) + 1 4y(y+1) + 1 . Since either y y or y + 1 y+1 must be even, the first term is divisible by 8 8 and the expression is equivalent to 1 1 (mod 8) \text{(mod 8)} .

Thus the possible values are 0 , 1 , 4 {0,1,4} , so there are 3 3

I just derped it.

Frodo Baggins - 7 years, 1 month ago

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Good job, Frodo. You have done well.

Josh Speckman - 7 years ago

I mean just square 0, you get 0. Square 1, you get 1. Square 2, you get 4. Square 3, you get 9 = 1. Square 4 you get 16 = 0. Square 5 you get 25 = 1. Square 6 you get 36 = 4. Square 7 you get 49 = 1. And that's all of them, so you just have 0, 1, 4.

Nate Ji - 7 years ago
William Isoroku
Jan 9, 2015

n : 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , . . . . . . n:4, 5, 6,7,8,9,10,11,......

x : 0 , 1 , 4 , 1 , 0 , 1 , 4 , 1 , 0 , . . . . . . . x:0,1,4,1,0,1,4,1,0,.......

The remainders, x x , are a pattern of 0 , 1 , 4 0,1,4 . So the answer is 3 \boxed{3}

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