Update 'backlog.md'

backlog.md

@@ -66,6 +66,7 @@
 - [Formula for Primes](https://en.wikipedia.org/wiki/Formula_for_primes)
 - [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)
 
 
 ## Reference Book Backlog