Imports System
' Additive primes: primes whose sum of digits is also prime
Module AdditivePrimes
' Check if a number is prime
Function IsPrime(n As UInteger) As Boolean
If n < 2UI Then Return False
If n Mod 2UI = 0UI Then Return n = 2UI
If n Mod 3UI = 0UI Then Return n = 3UI
Dim i As UInteger = 5UI
While i * i <= n
If n Mod i = 0UI Then Return False
i += 2UI
If n Mod i = 0UI Then Return False
i += 4UI
End While
Return True
End Function
' Compute the sum of digits of a number
Function SumDigits(n As UInteger) As UInteger
Dim sum As UInteger = 0UI
While n > 0UI
sum += n Mod 10UI
n \= 10UI
End While
Return sum
End Function
' Check if a number is an additive prime
Function IsAdditivePrime(n As UInteger) As Boolean
Return IsPrime(n) AndAlso IsPrime(SumDigits(n))
End Function
Sub Main()
Const TOP As UInteger = 500UI
Dim count As Integer = 0
For n As UInteger = 1UI To TOP - 1UI
If IsAdditivePrime(n) Then
Console.Write("{0,3}", n)
count += 1
If count Mod 10 = 0 Then
Console.WriteLine()
Else
Console.Write(" ")
End If
End If
Next
Console.WriteLine()
Console.WriteLine()
Console.WriteLine("Total additive primes = " & count)
End Sub
End Module
'run:
'
' 2 3 5 7 11 23 29 41 43 47
' 61 67 83 89 101 113 131 137 139 151
'157 173 179 191 193 197 199 223 227 229
'241 263 269 281 283 311 313 317 331 337
'353 359 373 379 397 401 409 421 443 449
'461 463 467 487
'
'Total additive primes = 54
'
