After a number removes its own factor, the sum of all other factors is equal to this number. A number like this is called a "perfect number". For example: the factors of 6 are 1, 2, 3, 6, except itself 6, the other 3 numbers are added together, 1+2+3=6. Some of the following numbers are also perfect numbers.
一个数除去本身这个因数后，其它所有因数的和等于这个数，像这样的数叫做"完全数"．例如：6的因数有1、2、3、6，除去它本身6外，其余3个数相加，1+2+3=6．下面几个数中，多少也是完全数．
<gadget id="calculator">1 + 2 + 4</gadget>
<output>7</output>

<gadget id="calculator">1 + 2 + 4 + 7 + 14</gadget>
<output>28</output>

<result>28</result>