+}
+
+@article{ref226,
+ title={Extending the Network Lifetime of Wireless Sensor Networks Using Residual Energy Extraction—Hybrid Scheduling Algorithm},
+ author={CHITRA, M},
+ journal={Int'l J. of Communications, Network and System Sciences},
+ volume={2010},
+ year={2010},
+ publisher={Scientific Research Publishing}
+}
+
+@article{ref227,
+ author = {Mihaela Cardei and
+ David MacCallum and
+ Maggie Xiaoyan Cheng and
+ Manki Min and
+ Xiaohua Jia and
+ Deying Li and
+ Ding-Zhu Du},
+ title = {Wireless Sensor Networks with Energy Efficient Organization},
+ journal = {Journal of Interconnection Networks},
+ volume = {3},
+ number = {3-4},
+ year = {2002},
+ pages = {213-229},
+}
+
+@inproceedings{ref228,
+ author = {D. Zorbas and D. Glynos and P. Kotzanikolaou and C. Douligeris},
+ title = {B\{GOP\}: an adaptive coverage algorithm for wireless sensor networks},
+ booktitle = {Proceedings of the 13th European Wireless Conference},
+ series = {EW'07},
+ year = {2007},
+ location = {Paris, France},
+
+}
+
+@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}
