Update 'backlog.md'

backlog.md

@@ -67,7 +67,8 @@
 - [Catalan Numbers](https://en.wikipedia.org/wiki/Catalan_number)
 - [Spinor](https://en.wikipedia.org/wiki/Spinor)
 - [Blumberg Theorem](https://en.wikipedia.org/wiki/Blumberg_theorem)
-
+- [Central Limit Theorem](https://en.wikipedia.org/wiki/Central_limit_theorem)
+- [Gershgorin circle theorem](https://en.wikipedia.org/wiki/Gershgorin_circle_theorem)
 
 ## Reference Book Backlog
 - Bartlett's Familiar Quotations