update book

doc/src/week36/exercisesweek36.do.txt

@@ -59,24 +59,15 @@ that $\vert\vert \bm{\beta}\vert\vert_2^2\le t$, where $t$ is
 a finite number larger than zero. We will not implement that here.
 
 === a) Expression for Ridge regression ===
+
 Show that the optimal parameters
 !bt
 \[
 \hat{\bm{\beta}}_{\mathrm{Ridge}} = \left(\bm{X}^T\bm{X}+\lambda\bm{I}\right)^{-1}\bm{X}^T\bm{y},
 \]
 !et
+with $\bm{I}$ being a $p\times p$ identity matrix.
 
-with $\bm{I}$ being a $p\times p$ identity matrix,
-
-with the constraint that
-
-!bt
-\[
-\sum_{i=0}^{p-1} \beta_i^2 \leq t,
-\]
-!et
-
-with $t$ a finite positive number. In the optimization, we will not require that the latter is satisfied.
 
 The ordinary least squares result is
 !bt