Imports System
Imports System.Collections.Generic
Module PrimeConcat
' Function to check primality
Function IsPrime(n As Long) As Boolean
If n < 2 Then Return False
If n Mod 2 = 0 AndAlso n <> 2 Then Return False
Dim i As Long = 3
While i * i <= n
If n Mod i = 0 Then Return False
i += 2
End While
Return True
End Function
' Concatenate two integers
Function Concat(a As Integer, b As Integer) As Long
Dim s As String = a.ToString() & b.ToString()
Return Long.Parse(s)
End Function
Sub Main()
Dim limit As Integer = 12 ' you can increase this
Dim primes As New List(Of Integer)()
' Generate primes up to limit
For i As Integer = 2 To limit
If IsPrime(i) Then
primes.Add(i)
End If
Next
' Check pairs
For i As Integer = 0 To primes.Count - 1
For j As Integer = 0 To primes.Count - 1
If i = j Then Continue For ' skip same prime if you want
Dim num As Long = Concat(primes(i), primes(j))
If IsPrime(num) Then
Console.WriteLine($"{primes(i)} + {primes(j)} -> {num} is prime")
End If
Next
Next
End Sub
End Module
' run:
'
' 2 + 3 -> 23 is prime
' 2 + 11 -> 211 is prime
' 3 + 7 -> 37 is prime
' 3 + 11 -> 311 is prime
' 5 + 3 -> 53 is prime
' 7 + 3 -> 73 is prime
' 11 + 3 -> 113 is prime
'
