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diff --git a/README.md b/README.md
index 3afa448..c21c383 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,3 @@
 - https://www.microsoft.com/en-us/research/uploads/prod/2018/03/trueskill2.pdf
+- https://www.microsoft.com/en-us/research/project/trueskill-ranking-system/
+- https://en.m.wikipedia.org/wiki/Glicko_rating_system