Editing Harmonic divisor number (section)

==Examples==
The number 6 has the four divisors 1, 2, 3, and 6. Their harmonic mean is an integer:
<math display="block"> \frac{4}{\frac{1}{1}+\frac{1}{2}+\frac{1}{3}+\frac{1}{6}}=2.</math>
Thus 6 is a harmonic divisor number.  Similarly, the number 140 has divisors 1, 2, 4, 5, 7, 10, 14, 20, 28, 35, 70, and 140. Their harmonic mean is
<math display="block">
 \frac{12}{\frac{1}{1}+\frac{1}{2}+\frac{1}{4}+\frac{1}{5}+\frac{1}{7}+\frac{1}{10}
+\frac{1}{14}+\frac{1}{20}+\frac{1}{28}+\frac{1}{35}+\frac{1}{70}+\frac{1}{140}}=5.
</math>
Since 5 is an integer, 140 is a harmonic divisor number.