Andrea Francke, Michael Hoffmann. The Euclidean degree-4 minimum spanning tree problem is NP-hard. In John Hershberger, Efi Fogel, editors, Proceedings of the 25th ACM Symposium on Computational Geometry, Aarhus, Denmark, June 8-10, 2009. pages 179-188, ACM, 2009. [doi]
@inproceedings{FranckeH09,
title = {The Euclidean degree-4 minimum spanning tree problem is NP-hard},
author = {Andrea Francke and Michael Hoffmann},
year = {2009},
doi = {10.1145/1542362.1542399},
url = {http://doi.acm.org/10.1145/1542362.1542399},
researchr = {https://researchr.org/publication/FranckeH09},
cites = {0},
citedby = {0},
pages = {179-188},
booktitle = {Proceedings of the 25th ACM Symposium on Computational Geometry, Aarhus, Denmark, June 8-10, 2009},
editor = {John Hershberger and Efi Fogel},
publisher = {ACM},
isbn = {978-1-60558-501-7},
}