' FreeBASIC v1.05.0 win64

Function SumProperDivisors(number As Integer) As Integer
  If number < 2 Then Return 0
  Dim sum As Integer = 0
  For i As Integer = 1 To number \ 2
    If number Mod i = 0 Then sum += i
  Next
  Return sum
End Function

Dim As Integer n, f
Dim As Integer sum(19999)

For n = 1 To 19999
  sum(n) = SumProperDivisors(n)
Next

Print "The pairs of amicable numbers below 20,000 are :"
Print

For n = 1 To 19998
  ' f = SumProperDivisors(n)
  f = sum(n)
  If f <= n OrElse f < 1 OrElse f > 19999 Then Continue For
  If f = sum(n) AndAlso n = sum(f) Then
    Print Using "#####"; n;
    Print " and "; Using "#####"; sum(n)
  End If
Next

Print
Print "Press any key to exit the program"
Sleep
End