Siregar, S., E. Soewono, D. Siregar, S. A. Putra, and Y. Budicakrayana. “Cost Minimization Model of Gas Transmission Line for Indonesian SIJ Pipeline Network”. Journal of Engineering and Technological Sciences, vol. 35, no. 1, Dec. 2013, pp. 23-43, doi:10.5614/itbj.eng.sci.2003.35.1.3.