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'Math - How to check whether given number is a Prime Number
Option Explicit
 
Public Function IsPrime(ByVal n As Long) As Boolean
    Dim I As Long
    IsPrime = False
    If n <> 2 And (n And 1) = 0 Then Exit Function 'test if div 2
    If n <> 3 And n Mod 3 = 0 Then Exit Function 'test if div 3
    For I = 6 To Sqr(n) Step 6
        If n Mod (I - 1) = 0 Then Exit Function
        If n Mod (I + 1) = 0 Then Exit Function
    Next I
    IsPrime = True
End Function