doi          = "10.1016/S0019-9958(67)90032-0",
 doi          = "10.1016/S0019-9958(75)90255-7",
 doi          = "10.1016/0304-3975(87)90124-1",
 doi          = "10.1016/S0019-9958(68)90309-4",
 doi          = "10.1016/S0019-9958(74)80055-0",
 doi          = "10.1016/0304-3975(82)90040-8",
 doi          = "10.1007/978-0-387-30440-3\_211",
 doi          = "10.1007/11560319\_20",
 doi          = "10.1007/978-3-540-31834-7\_18",
 doi          = "10.1016/S0019-9958(66)90110-0",