diff --git a/docs/links.md b/docs/links.md
index c9f7ec9..b3b9613 100644
--- a/docs/links.md
+++ b/docs/links.md
@@ -20,3 +20,6 @@ Interpolation
 Integration
 ===========
 - http://arxiv.org/pdf/1411.1341.pdf
+
+Hierarchial shape functions
+- https://www.math.vt.edu/people/adjerids/research/papers/basis.pdf
\ No newline at end of file