diff --git a/Engine-prototype.md b/Engine-prototype.md
index b0700c4..5305f63 100644
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+++ b/Engine-prototype.md
@@ -39,4 +39,8 @@ Some of these are obvious, but we may as well write them down. Note we may want
   * [Paper on deciding the existence of solutions](http://arxiv.org/abs/2106.08119)
   * [Relevant Math.StackExchange question](https://math.stackexchange.com/questions/2119007/how-do-you-solve-a-system-of-quadratic-equations)
 
-Feel free to add in other ideas in any of these categories.
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+Feel free to add in other ideas in any of these categories.
+
+## General references
+
+* Frank Sottile. *Real solutions to equations from geometry.*
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