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Question:- How many numbers are there between 1 to 1000 which can be expressed by the difference of two square numbers?

Pls help to solve it.i have used brute force but getting wrong answer:(
Here is my Code in C++

The output is 327,as there is repetition i have used set to store the unique numbers only.

#include<iostream>
using namespace std;
int main()
{
int a[32];
set<int>s;
for(int i = 1;i <= 31;i++){
a=i*i;
}

for(int i = 1;i <= 30;i++){
for(int j = i+1;j <= 31;j++){
s.insert(abs(a-a[j]));
cout<<abs(a-a[j])<<endl;
}
}
cout << s.size() <<endl;
}
}

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Every odd number is difference of two squares, $2n+1=(n+1)^2-n^2$

Every even number that's divisible by $4$ is difference of two squares, $4n=(n+1)^2-(n-1)^2$.

Every number of the form $4n+2$ can't be a difference of two squares, for the sake of contradiction, let's assume,
$4n+2=a^2-b^2=(a+b)(a-b)$
Now, $a+b\equiv a-b\pmod{2}$
So, $a+b,a-b$ both must be even. Therefore, $4\mid (a+b)(a-b)$.
But $4\nmid 4n+2$, which is a contradiction.

So, use this code instead,