Added references

Author: MarkTheHopeful

Diff for: docs/src/md/kotlin.core/declarations.md

@@ -1826,6 +1826,10 @@ This can be interpreted in the following way:
 - Contravariance *inverses* the given variance
 - Invariance *absorbs* the given variance
 
+Further reasoning and formalization behind this system is available in the ["Taming the Wildcards" paper][declarations.references].
+
+[declarations.references]: #declarations.references
+
 Using the base not-nested cases and the rules for combining variance, we can iteratively define for every type expression its variance in a given type parameter.
 
 When a type parameter has declared variance, and is used in a position of a different variance, a **variance conflict** may occure. The specific cases are:
@@ -2067,3 +2071,7 @@ There is a partial order of *weakness* between different visibility modifiers:
 > ```
 > 
 
+### References {#declarations.references}
+
+1. John Altidor, Shan Shan Huang, and Yannis Smaragdakis. "Taming the wildcards: combining definition- and use-site variance." 2011 (\url{https://yanniss.github.io/variance-pldi11.pdf})
+