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I don't know a reference. One way to show the eigenvalues starts from the observation (which can be proved using generating functions) that $\sum_{i=0}^n {i\choose k} A_{i,j}={2n+1 \choose n-k} {j+k \choose k}={2n+1 \choose n-k}\,\sum_{\ell=0}^k {k\choose \ell} { j \choose \ell}$ (where $A$ is the matrix above). With the row vectors $\mathbf{v}_k$ with coordinates $\mathbf{v}_k(i)={i \choose k}$ that is $$\mathbf{v}_k A={2n+1 \choose n-k}\left(\sum_{\ell=0}^k {k \choose \ell} \mathbf{v}_\ell\right).$$ The rest is routine.