doi          = {10.1007/978-3-642-33263-0\_31},
 doi          = {10.1080/10867651.1997.10487468},
 doi          = {10.1017/s0956796897002967},
 doi          = {10.22478/ufpb.2595-0622.2017v1n1.37497},
 doi          = {10.1007/11894100\_8},