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The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

I couldn't use the AllPrimes function I had developed for some
other Project Euler problems since the maximum array size in VB ( 2^{31}-1)
is not large enough for the number in this problem. Luckily,
the number under consideration has less than 15 significant digits
and hence it will fit in a Double data type.

So, I wrote a long overdue routine 'IsPrime,' which returns True if a number is a prime number. The code below ran in about 0.08 seconds on my laptop.

Option Explicit Function IsPrime(aNbr As Variant) As Boolean Dim K As Long K = 1 If aNbr Mod 2 = 0 Or aNbr Mod 3 = 0 Then IsPrime = False: Exit Function Do While 6 * K - 1 < Sqr(aNbr) If aNbr Mod (6 * K - 1) = 0 Or aNbr Mod (6 * K + 1) = 0 Then _ IsPrime = False: Exit Function K = K + 1 Loop IsPrime = True End Function Sub Euler003() Dim aNbr As Double: aNbr = 600851475143 'aNbr = CDec(999999) Dim I As Double, ProcTime As Single ProcTime = Timer I = CDec(Int(Sqr(aNbr))) Do While I > 1 If aNbr / I = Int(aNbr / I) Then If IsPrime(I) Then Exit Do I = I - 1 Loop Debug.Print I, Timer - ProcTime End Sub