I am working with Java and trying to write a very simple test code like this to test and I thought that there was no problem but there was one problem.  public class EmployeeTest {     public static void main(String[] args){         Employee[] staff = new Employee[1];         staff[0] = new Employee(&quot;Carl Cracker&quot;, 75000);         for (Employee e : staff)             System.out.println(&quot;name=&quot; + e.getName() + &quot;,salary=&quot; + e.getSalary());     }      class Employee {     private String name;     private double salary;     public Employee(String n, double s)     {         name = n;         salary = s;     }     public String getName()     {         return name;     }     public double getSalary()     {         return salary;     } } }  Unfortunately, when I ran my code, the result is No enclosing instance of type EmployeeTest is accessible. Must qualify the allocation with an enclosing instance of type EmployeeTest (e.g. x.new A() where x is an instance of EmployeeTest).  I am sure that my code is very simple and easy to understand, so I am very confusing that why I had that error? Could you please give me some comments ? Thank you very much!
What is the main difference between an inner class and a static nested class in Java? Does design / implementation play a role in choosing one of these?
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For all $X \geq 1$,$\sum\limits_{1 \leq n \leq X} \mu(n) \left[\frac{X}{n}\right] = 1.$ I'm currently sitting with the following number theory problem:  Prove that for all $X \geq 1$, $$\sum_{1 \leq n \leq X} \mu(n) \left[\frac{X}{n}\right] = 1.$$  A few ideas I have tried: Using that $[x]=x - \{x\}$ which I couldn't make out to bring anything. Switching around on the summation variables, like $$\sum_{d \leq N} \mu(d) f\left( \frac{n}{d} \right) = \sum_{d \leq N} \mu \left( \frac{d}{n} \right) f(d)$$ but that didn't seem to bring anything.  I tried writing out the first few terms, which give $$\mu(1) [X] - \left[\frac{X}{2}\right] - \left[\frac{X}{3}\right] - \left[\frac{X}{5}\right] + \left[\frac{X}{6}\right] - \left[\frac{X}{7}\right] - \left[\frac{X}{8}\right]$$ but I can't seem to find a system where it simply reduces to one!  What do you think?