# Strong rules for the elastic net

In their paper (here), Tibshirani et al defined the lasso as the solution to $$\text{argmin}_{\boldsymbol{\beta}}\frac{1}{2}\left\Vert \mathbf{y}-\mathbf{X}\boldsymbol{\beta}\right\Vert ^{2}+\lambda\left\Vert \boldsymbol{\beta}\right\Vert _{1}$$ and give the sequential strong rule for the lasso coefficients being zero as $$\left|\mathbf{x}_{j}^{T}\left(\mathbf{y}-\mathbf{X}\hat{\boldsymbol{\beta}}\left(\lambda_{k-1}\right)\right)\right|<2\lambda_{k}-\lambda_{k-1}$$ with $$\lambda_{0}=\lambda_{\text{max}}=\max_{j}\left|\mathbf{x}_{j}^{T}\mathbf{y}\right|$$.

The elastic net is a special form of the lasso as $$\text{argmin}_{\boldsymbol{\beta}}\frac{1}{2}\left\Vert \tilde{\mathbf{y}}-\tilde{\mathbf{X}}\boldsymbol{\beta}\right\Vert ^{2}+\lambda\alpha\left\Vert \boldsymbol{\beta}\right\Vert _{1}$$ with $$\tilde{\mathbf{X}}=\begin{pmatrix}\mathbf{X}\\ \sqrt{\lambda\left(1-\alpha\right)}\mathbf{I} \end{pmatrix},\quad\tilde{\mathbf{y}}=\begin{pmatrix}\mathbf{y}\\ \mathbf{0} \end{pmatrix}$$ They give the strong rules for the elastic net as $$\left|\mathbf{x}_{j}^{T}\left(\mathbf{y}-\mathbf{X}\hat{\boldsymbol{\beta}}\left(\lambda_{k-1}\right)\right)\right|<\alpha\left(2\lambda_{k}-\lambda_{k-1}\right)$$

My problem:

When I put the expressions of $$\tilde{\mathbf{X}}$$ and $$\tilde{\mathbf{y}}$$ into the strong rule of the lasso, I could not get this expression of strong rule of elastic net.

Instead, I got $$\left|\mathbf{x}_{j}^{T}\left(\mathbf{y}-\mathbf{X}\hat{\boldsymbol{\beta}}\left(\lambda_{k-1}\right)\right)+\lambda\left(1-\alpha\right)\mathbf{x}_{j}^{T}\hat{\boldsymbol{\beta}}\left(\lambda_{k-1}\right)\right|<\alpha\left(2\lambda_{k}-\lambda_{k-1}\right)$$ Where is my mistake?