4. (romanian national olympiad, 10th grade 2000) How many ways can you tile a 3xn rectangle with dominoes.
5.
[code]

  for i[1] = 1 to n
    for i[2] = i[1] to n
      for i[3] = i[2] to n

  for i1 = 1 to n
    for i2 = i1 to n
      for i3 = i2 to n

        …

         for i[k] = i[k - 1] to  n

         for ik = ik - 1 to  n

             print ‘*’
[/code]
How many stars will be printed for a given n and k.