The application of the square theorem is analyzed

We can verify its correctness in limited range by computer.

#include <stdio.h>
#include <math.h>
int f(int n, int a[], int idx)
{
 if(n==0) return 1;  
 if(idx==4)  return 0;  //Error exceeding array range
 for(int i=(int)sqrt(n); i>=1; i--)
 {
  a[idx] = i;
  if(f(n-i*i, a, idx+1) == 1)  return 1;  //Recursive calls
 }
 return 0;
}
int main(int argc, char* argv[])
{
 for(;;)
 {
  int number;
  printf(" Enter an integer (1~10 Hundred million ) : ");
  scanf("%d",&number);

  int a[] = {0,0,0,0};
  int r = f(number, a, 0);
  printf("%d: %d %d %d %dn", r, a[0], a[1], a[2], a[3]);

 }
 return 0;
}