U.S. telephone numbers, including the NPA (area code), contain 10 digits.

The largest area code in the U.S. is 989, which is in Michigan. This part is called the NPA, the second digit of which cannot currently be a 9; in the future, when the current system becomes inadequate, use of a 9 in this position may be allowed. Next is the NXX. This can be as high as 999. The remaining four digits can be as high as 9999.

see:

The largest prime meeting the criteria, formatted as a phone number, is 989-999-9987.

DefDbl A-Z

Dim crlf$

Function mform$(x, t$)

  a$ = Format$(x, t$)

  If Len(a$) < Len(t$) Then a$ = Space$(Len(t$) - Len(a$)) & a$

  mform$ = a$

End Function

Private Sub Form_Load()

 Text1.Text = ""

 crlf$ = Chr(13) + Chr(10)

 Form1.Visible = True

 

 p = 9899999999#

 Text1.Text = prvprm(p) & crlf

 

 Text1.Text = Text1.Text & "done"

 

 End Sub

 

 Function prmdiv(num)

 Dim n, dv, q

 If num = 1 Then prmdiv = 1: Exit Function

 n = Abs(num): If n > 0 Then limit = Sqr(n) Else limit = 0

 If limit <> Int(limit) Then limit = Int(limit + 1)

 dv = 2: GoSub DivideIt

 dv = 3: GoSub DivideIt

 dv = 5: GoSub DivideIt

 dv = 7

 Do Until dv > limit

   GoSub DivideIt: dv = dv + 4 '11

   GoSub DivideIt: dv = dv + 2 '13

   GoSub DivideIt: dv = dv + 4 '17

   GoSub DivideIt: dv = dv + 2 '19

   GoSub DivideIt: dv = dv + 4 '23

   GoSub DivideIt: dv = dv + 6 '29

   GoSub DivideIt: dv = dv + 2 '31

   GoSub DivideIt: dv = dv + 6 '37

 Loop

 If n > 1 Then prmdiv = n

 Exit Function

DivideIt:

 Do

  q = Int(n / dv)

  If q * dv = n And n > 0 Then

    prmdiv = dv: Exit Function

   Else

    Exit Do

  End If

 Loop

 Return

End Function

 

Function prvprm(x)

  Dim n

  n = x

  While prmdiv(n) < n Or n < 2

    n = n - 2

    DoEvents

  Wend

  prvprm = n

End Function