}
+@inproceedings{ref229,
+ title={A near optimal algorithm for lifetime optimization in wireless sensor networks},
+ author={Deschinkel, Karine and Hakem, Mourad},
+ booktitle={SENSORNETS 2013, 2nd Int. Conf. on Sensor Networks},
+ pages={197--202},
+ year={2013}
+}
+
+@article{ref230,
+ title={Maximizing system lifetime in wireless sensor networks},
+ author={Alfieri, Arianna and Bianco, Andrea and Brandimarte, Paolo and Chiasserini, Carla-Fabiana},
+ journal={European Journal of Operational Research},
+ volume={181},
+ number={1},
+ pages={390--402},
+ year={2007},
+ publisher={Elsevier}
+}
+
+@inproceedings{ref231,
+ title={Maximize lifetime of heterogeneous wireless sensor networks with joint coverage and connectivity requirement},
+ author={Gu, Yu and Ji, Yusheng and Zhao, Baohua},
+ booktitle={Scalable Computing and Communications; Eighth International Conference on Embedded Computing, 2009. SCALCOM-EMBEDDEDCOM'09. International Conference on},
+ pages={226--231},
+ year={2009},
+ organization={IEEE}
+}
\ No newline at end of file
