@article{BTW87,
 author = {P. Bak, and C. Tang, and K. Wissenfeld},
 title = {Self-Organized Criticality: An Explanation of $ 1/f $ Noise},
 journal = {Phys. Rev. Lett},
 volume = {{\bf 59}},
 number = {4},
 year = {1987},
}
@article{BTW88,
 author = {T. Hwa and M. Kardar},
 title = {Dissipative transport in open systems: An investigation of self-organized criticality},
 journal = {Phys. Rev. Lett.},
 volume = {62},
 number = {1813},
 year = {1989},
 }