1. Laurent Alfandari and Vangelis Th Paschos. 1999. Approximating minimum spanning tree of depth 2. International Transactions in Operational Research 6, 6 (1999), 607--622.
2. Eric Angel, Evripidis Bampis, and Laurent Gourvés. 2004. Approximating the Pareto curve with local search for the bicriteria TSP (1, 2) problem. Theoretical Computer Science 310, 1--3 (2004), 135--146.
3. AE Baburin, Federico Della Croce, E Kh Gimadi, Yu V Glazkov, and V Th Paschos. 2009. Approximation algorithms for the 2-peripatetic salesman problem with edge weights 1 and 2. Discrete Applied Mathematics 157, 9 (2009), 1988--1992.
4. Piotr Berman and Marek Karpinski. 2006. 8/7-approximation algorithm for (1, 2)-TSP. In Proceedings of the seventeenth annual ACM-SIAM symposium on Discrete algorithm. Society for Industrial and Applied Mathematics, 641--648.
5. Marshall Bern and Paul Plassmann. 1989. The Steiner problem with edge lengths 1 and 2. Inform. Process. Lett. 32, 4 (1989), 171--176.