PROGRAM  Poisson_equation_in_2D  parameter  (kx= 40 ,ky= 40 )  integer ::i,j,k  real ::rho(kx,ky),V(kx,ky),R(kx,ky),d  open ( 1 , file ='poisson input', status ='unknown')  open ( 2 , file ='poisson output', status ='unknown')  read ( 1 ,*)((rho(i,j),j= 1 ,ky),i= 1 ,kx)  k= 0   d= 0 . 025   do  i= 1 ,kx  do  j= 1 ,ky  V(i,j)= 0 . 0   enddo   enddo   4  do  i= 2 ,kx- 1   do  j= 2 ,ky- 1   R(i,j)=V(i+ 1 ,j)+V(i- 1 ,j)+V(i,j+ 1 )+V(i,j- 1 )- 4 *V(i,j)+rho(i,j)*d** 2   enddo   enddo   do  i= 2 ,kx- 1   do  j= 2 ,ky- 1   if ( abs (R(i,j))> 0 . 0000001 ) goto  3   enddo   enddo   goto  5   3  k=k+ 1   do  i= 2 ,kx- 1   do  j= 2 ,ky- 1   V(i,j)=V(i,j)+ 0 . 25 *R(i,j)  enddo   enddo   goto  4   5  write (*, 6 )k  do  i= 1 ,kx+ 1   do  j= 1 ,ky+ 1   write ( 2 , 7 )i,j,V(i,j)  enddo   enddo   6  format ('no of Iteration=',i5)  7  format (i2,1x,i2,1x,f7. 5 )  end