[数理统计] 验证哥德巴赫猜想Fortran程序初探

module SuShu
  implicit none
  integer :: even(2), odd(3) 
  private :: isprime
Contains
! ---------------------------------------------------------------
  logical function fenjie( x )
    implicit none
    integer,intent(in) :: x
    integer :: i,j,k
    fenjie  = .false. 
    if( mod(x, 2) .eq. 0 ) then 
      do i = 2, x
        if( .not.isprime( i ) ) cycle
        do j = 2, x
          if( .not.isprime( j ) ) cycle
          if( (i+j - x) .eq. 0 ) then 
            even(1) = i
            even(2) = j 
            fenjie  = .true. 
            return 
          end if 
        end do 
      end do 
    else 
      do i = 2, x
        if( .not.isprime( i ) ) cycle
        do j = 2, x
          if( .not.isprime( j ) ) cycle
          do k = 2, x
            if( .not.isprime( k ) ) cycle
            if( (i+j+k - x) .eq. 0 ) then 
              odd(1) = i
              odd(2) = j
              odd(3) = k
              fenjie = .true. 
              return 
            end if 
          end do 
        end do 
      end do 
    end if 

  end function fenjie
! ---------------------------------------------------------------

! ---------------------------------------------------------------
  logical function isprime(a)
    implicit none 
    integer,intent(in) :: a
    integer :: i  
    if( (a .eq. 2) ) then 
      isprime = .true.
      return 
    end if 
    do i = 2,  int( sqrt( a*1. ) ) + 1
      if( mod(a,i) .eq. 0) then 
        isprime = .false.
        return 
      end if 
    end do 
    isprime = .true.
    return 
  end function isprime
! ---------------------------------------------------------------
end module SuShu

program main
  use SuShu
  implicit none
  integer :: n,i
  open(11, file = "odd_number_results.txt")   !  奇数结果
  open(12, file = "even_number_results.txt")  !  偶数结果

  do i = 6, 50 ! 验证范围
    if ( fenjie(i) ) then 
      if( mod(i, 2) .eq. 0 ) then 
        write(12,100) i," = ",even(1)," + ",even(2) 
      else 
        write(11,101) i," = ",odd(1)," + ",odd(2)," + ",odd(3)
      end if 
    else 
      if( mod(i, 2) .eq. 0 ) then 
        write(12,*) "No. ",i," does not satisfy Goldbach's conjecture"
      else 
        write(11,*) "No. ",i," does not satisfy Goldbach's conjecture"
      end if 
    end if 
  end do
  
  close(11)
  close(12)
  100 format( i3,a,i3,a,i3 ) 
  101 format( i3,a,i3,a,i3,a,i3 ) 
end program main