do i=1,4000
select case (i)
case (1:9)
write(form,'(i1)') i
case (10:99)
write(form,'(i2)') i
case (100:999)
write(form,'(i3)') i
case (1000:9999)
write(form,'(i4)') i
end select
write(filename,*) "AA",trim(form),".TXT"
open(10,file=filename)
write(10,*) i
close(10)
end do