求1000到4000内满足这样条件的数之和:这个数的所有因子之和能够同时被3和7整除

很难用数学方法做.
共有721个这样的数,总和1810606
VB不严密代码：
he = 0
for i= 1000 to 4000
iysh = 1
for k = 2 to i
if i mod k = 0 then
iysh = iysh + k
end if
next
if iysh mod 21 = 0 then
he = he + i
end if
next
print he