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# Diophantine equation

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Show that the Diophantine equation x^2-y^2=n is solvable in integers if and only if n is odd or n is divisible by 4. When this equation is solvable, find all integer solutions.

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This show show to prove that a Diophantine equation is solvable in integers if and only if certain circumstances are met.

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Show that the Diophantine equation x^2-y^2=n is solvable in integers if and only if n is odd or n is divisible by 4. When this equation is solvable, find all integer solutions.

First, we will prove the statement:

x^2-y^2=n is solvable in integers if and only if n is odd or n is divisible by 4.

We need to show that "if x^2-y^2=n is solvable in integers, then n is odd or n is divisible by 4"
Proof. We know that and have the same parity since (x+y)+(x-y)=2x is even. Since x^2-y^2=n is solvable in integers and , we have
Case 1: x+y is odd
Then x-y must be odd. So, is odd.
Case 2: x+y is even
Assume that . Then x-y must be even, say . So,
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• BSc , Wuhan Univ. China
• MA, Shandong Univ.
###### Recent Feedback
• "Your solution, looks excellent. I recognize things from previous chapters. I have seen the standard deviation formula you used to get 5.154. I do understand the Central Limit Theorem needs the sample size (n) to be greater than 30, we have 100. I do understand the sample mean(s) of the population will follow a normal distribution, and that CLT states the sample mean of population is the population (mean), we have 143.74. But when and WHY do we use the standard deviation formula where you got 5.154. WHEN & Why use standard deviation of the sample mean. I don't understand, why don't we simply use the "100" I understand that standard deviation is the square root of variance. I do understand that the variance is the square of the differences of each sample data value minus the mean. But somehow, why not use 100, why use standard deviation of sample mean? Please help explain."
• "excellent work"
• "Thank you so much for all of your help!!! I will be posting another assignment. Please let me know (once posted), if the credits I'm offering is enough or you ! Thanks again!"
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