Combinatorics terms

docs/Public/Math/LaTeX cheat sheet.md

@@ -60,3 +60,5 @@
 `\displaystyle`: Make math expressions large
 
 `\approx`: $\approx$
+
+`n \choose x`: $n \choose x$

docs/Public/Math/bijection.md

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+A [[function]] is a *bijection* when it is both [[onto]] and [[one to one]].
+
+Translation: Each element $a \in A$ is matched with a unique element $f(a) \in B$, and every element of $B$ is matched with something from $A$. 
+

docs/Public/Math/one to one.md

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+A [[function]] is *one-to-one* if 
+
+$$
+f(a) = f(b) \implies a = b
+$$
+Another way:
+
+$$
+a \not = a' \implies f(a) \not = f(a')
+$$
+Translation: Distinct inputs give distinct outputs, no two arrays have the same head

docs/Public/Math/onto.md

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+A [[function]] is *onto* if for every $y$, there is an $x$ such that $f(x) = y$.
+
+For each $b \in B$ there exists at least one $a \in A$ such that $f(a) = b$.
+
+Translation: Every element of $B$ is "pointed to"