All even perfect numbers have the form
 (2^p-1) * 2^(p-1) with p prime one of this set:

        2          3          5           7
       13         17         19          31
       61         89        107         127
      521        607       1279        2203
     2281       3217       4253        4423
     9689       9941      11213       19937
    21701      23209      44497       86243
   110503     132049     216091      756839
   859433    1257787    1398269     2976221
  3021377