Public Static Function PE4_Karcrack(Optional ByVal lCifras As Long = 3) As Double
    Dim A   As Double
    Dim B   As Long
    Dim C   As Long
    Dim D   As Long
    Dim M   As Long
    Dim R   As Double
 
    B = (10 ^ lCifras) - 1
    M = (10 ^ (lCifras - 1))
 
    Do Until (M Mod 11) = 0
        M = M + 1
    Loop
 
    D = B
 
    Do Until (D Mod 11) = 0
        D = D - 1
    Loop
 
    For A = D To M Step -11
        For C = B To A * 0.8 Step -1
            R = (A * C)
            If R <= PE4_Karcrack Then Exit For
            If R = NReverse(R) Then
                PE4_Karcrack = R
            End If
        Next C
    Next A
End Function
 
 
Public Static Function NReverse(ByVal D As Double) As Double
    Dim dig As Long
    While D > 0
        dig = (D - Fix(D / 10#) * 10)
        NReverse = NReverse * 10 + dig
        D = Fix(D / 10)
    Wend
End Function

Private Sub Form_Load()
    Dim i   As Long
 
    For i = 2 To 5
        Debug.Print i, PE4_Karcrack(i)
    Next i
End Sub

 
  2             9009 
 3             906609 
 4             99000099 
 5             9966006699 
 6             999000000999