diff --git a/Engine-prototype.md b/Engine-prototype.md
index a9e5424..969c7a6 100644
--- a/Engine-prototype.md
+++ b/Engine-prototype.md
@@ -43,4 +43,4 @@ Feel free to add in other ideas in any of these categories.
 
 ## General references
 
-* Frank Sottile. *Real solutions to equations from geometry.* (See https://franksottile.github.io/teaching/10.S/RSEG.html)
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+* Frank Sottile. [*Real solutions to equations from geometry*](https://franksottile.github.io/tmp/sottile.pdf). (See https://franksottile.github.io/teaching/10.S/RSEG.html)
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