proving $\binom{n-1}{k} - \binom{n-1}{k-2} = \binom{n}{k} - \binom{n}{k-1} $ The identity is
$$\binom{n-1}{k} - \binom{n-1}{k-2} = \binom{n}{k} - \binom{n}{k-1}. $$
I wrote a matlab code and numerically checked it. It is right.
It is also not difficult to prove it by brute force. But can anyone come up with a simple proof using combinatorics? Many such identities are proven in this elegant way.
ps. It is an identity in the 'Group theory' book by Wigner.
 A: We will prove the equivalent $$\binom{n-1}k+\binom n{k-1}=\binom nk +\binom{n-1}{k-2}\tag1$$
Let $[m]=\{1,2,3,\dots,m\}.$ Write $$\binom{[m]}j=\{S\subseteq [m]\mid |S|=j\}$$ Let $$\mathcal S_1=\binom{[n-1]}k\cup \binom{[n]}{k-1}\\\mathcal S_2=\binom{[n]}k\cup \binom{[n-1]}{k-2}$$
Show $|\mathcal S_1|,|\mathcal S_2|$ are equal to the left and right side of (1), respectively.
The map sending $f:\mathcal S_1\to\mathcal S_2$ defined as: $$f(S)=\begin{cases}
S\setminus \{n\}& |S|=k-1,n\in S\\
S\cup\{n\}& |S|=k-1, n\not\in S\\
S&|S|=k
\end{cases}$$
The image in the first case is a subset of $[n-1]$ of size $k-2.$
The image in the other two cases is a subset of $[n]$ of size $k.$
Show this is one-to-one and onto.

This is really just hiding the direct proof using the simpler combinatorial result: $$\binom{n}{k}=\binom{n-1}k+\binom{n-1}{k-1}\\\binom n{k-1}=\binom{n-1}{k-1}+\binom{n-1}{k-2}$$
Both sides of $(1)$ are equal to the total number of subsets of $[n-1]$ of sizes $k-2,k-1,$ or $k.$
A: Hint: You might also find appealing that algebraically this binomial identity is encoded by the relation
\begin{align*}
1-z^2=(1+z)(1-z)
\end{align*}
Denoting with $[z^k]$ the coefficient of $z^k$ of a series we can write
\begin{align*}
\binom{n}{k}=[z^k](1+z)^n\tag{1}
\end{align*}

Using (1) and noting that $[z^p]z^qA(z)=[z^{p-q}]A(z)$ we obtain
\begin{align*}
\color{blue}{\binom{n-1}{k}}\color{blue}{ - \binom{n-1}{k-2}}
&=[z^k](1+z)^{n-1}-[z^{k-2}](1+z)^{n-1}\\
&=[z^k]\left(1-z^2\right)(1+z)^{n-1}\\
&=[z^k](1-z)(1+z)^n\\
&=[z^k](1+z)^n-[z^{k-1}](1+z)^{n}\\
&\,\,\color{blue}{=\binom{n}{k}-\binom{n}{k-1}}
\end{align*}

