Lucas Lehmer test question?

[BMgf]

I don't know way, but this test doesn't working when I input 31 in txt1.(it works with smaller prime numbers)

        Dim P As Int64        'Int64 means 64-bit integer
        Dim S As Int64
        Dim I As Int64
        P = CInt(txt1.Text)     'This converts txt1 Text into P-Integer
        S = 4
        For I = 3 To M
            S = ((S ^ 2) - 2) Mod ((2 ^ P) - 1)
        Next
        If S = 0 Then
            MsgBox("It's a prime")
        Else
            MsgBox("It's not a prime")
        End If

Any help?
Thanks in advance!

[andi314]

which value did you asign to M??

For I = 3 To M

so the loop will never end

[BMgf]

M=P

        Dim P As Int64        'Int64 means 64-bit integer
        Dim S As Int64
        Dim I As Int64
        P = CInt(txt1.Text)     'This converts txt1 Text into P-Integer
        S = 4
        For I = 3 To P
            S = ((S ^ 2) - 2) Mod ((2 ^ P) - 1)
        Next
        If S = 0 Then
            MsgBox("It's a prime")
        Else
            MsgBox("It's not a prime")
        End If

I made mistake in retyping!

[andi314]

maybe the size for your number are too small???

[BMgf]

Int64 can hold numbers up to 2^63 so, it shouldn't be the problem. Also, I don't get any error message like I get with i.e. P = 63 (overflow).
Any other idea?

[andi314]

maybe the modulo function doesnt work with such hich number??

[BMgf]

maybe!?!

[andi314]

which programming language do you use???

[BMgf]

Microsoft Visual Basic.NET

[andi314]

do you have any manual where you can look up, how far the mod function works???

[BMgf]

It's not problem in that!

        Dim P As Int64
        Dim S As Int64
        Dim M As Int64
        Dim I As Int64
        P = CInt(txt1.Text)
        S = 4
        M = (2 ^ P) - 1
        For I = 3 To P
            S = (S ^ 2) - 2
            While S >= M
                S = S - ((S \ M) * M)   ' \ - it divides S with M   i.e. 5\2 = 2
            End While
        Next
        If S = 0 Then
            MsgBox("It's a prime")
        Else
            MsgBox("It's not a prime")
        End If

I replaced mod function with this and it's still the same