• Product Rule: finite set $|S\times T|=|S||T|$
• Sum Rule: finite disjoint set $|S\cup T|=|S|+|T|$
• Bijection Rule: finite set $\exists\phi:S\rightarrow T$ onto and 1-1 then $|S|=|T|$
  • 1-1: $\leq$
  • onto: $\geq$
• $[m] = {0,1,2,\cdots,m-1}$
• $2^{[m]}={S|S\subseteq [n]}$: Power set
• ${n\choose k}=|{[n]\choose k}|$
  • ${S\choose k}={T\subseteq S||T|=k}$
• ${n\choose m_1,\cdots,m_k}=\frac{n!}{m_1!\cdots m_k!}$
  • number of assignments such that $i$-th bin has $m_i$ balls in it
  • number of permutations of a multiset $M$ with $|M|=n$ such that $M$ has $k$ distinct elements whose multiplicities are given by $m_1,m_2,\cdots,m_k$
• $(m)_n= {m\choose m-n}$: $m$ lower factorial $n$
• Binominal theorem: $(1+x)^n=\sum_{k=0}^n{n\choose k}x^k$
• Multinominal theorem: $(x_1+\cdots+x_k)^n=\sum_{m_1+\cdots+m_k=n}{n\choose m_1,\cdots,m_k}x_1^{m_1}\cdots x_k^{m_k}$
• $k$-composition of $n$: ${n-1\choose k-1}$
  • ordered sum of positive integers
• week $k$-composition of $n$: ${n+k-1\choose k-1}$
  • number of nonnegative solutions to $x_1+x_2+\cdots+x_k\leq n$: ${n+k \choose k}$
• $k$-multisets on $S$: $({n\choose k})={n+k-1\choose n-1}={n+k-1\choose k}$
• ${n\brace k}$: $k$-partitions of an $n$-set, Stirling number of the second kind
  • ${n\brace k}=k{n-1\brace k}+{n-1 \brace k-1}$
  • $\left{{n \atop m}\right}={\frac {1}{m!}}\sum _{k=1}^{m}(-1)^{m-k}{m \choose k}k^{n}$
• $B(n)=\sum_{k=1}^n{n\brace k}$: partitions of an $n$-set, Bell number
• $p_k(n)$: Partitions of a number, a multiset ${x_1,\cdots,x_k}$ with $x_i\geq 1$ and $\sum_{i=1}^kx_i=n$
  • $p_k(n)=p_{k-1}(n-1)+p_k(n-k)$
  • $p_k(n)\sim\frac{n^{k-1}}{k!(k-1)!},n\rightarrow\infty$
    • ${n-1\choose k-1}\leq k!p_k(n)\leq {n+\frac{k(k-1)}{2}-1\choose k-1}$
• $p(n)=\sum_{k=1}^np_k(n)$: partition number
• Ferrers diagram(Young diagram)
  • Conjugate partition
  • The number of partitions of $n$ which have largest summand $k$ is $p_k(n)$
  • $p_k(n)=\sum_{j=1}^kp_j(n-k)$

Function $f:N\rightarrow M$