The implementations are based on various publications; if you find references like [KK89], they refer to papers which are listed below.
[Ead84] Peter Eades. A heuristic for graph drawing. Congressus Numerantium, 42:149–160, 1984.
[Kou13] Kobourov, S. G. (2013). Handbook of Graph Drawing and visualization. (R. Tamassia, Ed.). CRC Press, an imprint of Chapman and Hall/CRC.
[FR91] Fruchterman, T.M.J. and Reingold, E.M. (1991), Graph drawing by force-directed placement. Softw: Pract. Exper., 21: 1129-1164. https://doi.org/10.1002/spe.4380211102
[KK89] Kamada and S. Kawai, “An Algorithm for Drawing General Undirected Graphs”, Information Processing Letters 31 (1989), 7–15
[HK01] Harel, D., Koren, Y. (2001). A Fast Multi-scale Method for Drawing Large Graphs. In: Marks, J. (eds) Graph Drawing. GD 2000. Lecture Notes in Computer Science, vol 1984. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-44541-2_18
This repo can be used under either the MIT or Apache 2.0 license (see LICENSE-MIT and LICENSE-APACHE), according to your choice.