added about huber cost function

doc/pub/How2ReadData/html/How2ReadData-bs.html

@@ -1203,7 +1203,10 @@ years etc.
 <p>
 Finally, another cost function is the Huber cost function used in robust regression.
 It is less sensitive to outliers in data than the squared error cost function.
-A variant for classification is also sometimes used, a quantity we will meet later.
+A variant for classification is also sometimes used, a quantity we will meet later. 
+$$
+L_{\delta }(a)={\begin{cases}{\frac {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}}L_{\delta }(a)={\begin{cases}{\frac  {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac  {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}
+$$
 
 <p>
 We will discuss in more

doc/pub/How2ReadData/html/How2ReadData-reveal.html

@@ -1228,7 +1228,12 @@ years etc.
 <p>
 Finally, another cost function is the Huber cost function used in robust regression.
 It is less sensitive to outliers in data than the squared error cost function.
-A variant for classification is also sometimes used, a quantity we will meet later.
+A variant for classification is also sometimes used, a quantity we will meet later. 
+<p>&nbsp;<br>
+$$
+L_{\delta }(a)={\begin{cases}{\frac {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}}L_{\delta }(a)={\begin{cases}{\frac  {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac  {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}
+$$
+<p>&nbsp;<br>
 
 <p>
 We will discuss in more

doc/pub/How2ReadData/html/How2ReadData-solarized.html

@@ -1163,7 +1163,10 @@ years etc.
 <p>
 Finally, another cost function is the Huber cost function used in robust regression.
 It is less sensitive to outliers in data than the squared error cost function.
-A variant for classification is also sometimes used, a quantity we will meet later.
+A variant for classification is also sometimes used, a quantity we will meet later. 
+$$
+L_{\delta }(a)={\begin{cases}{\frac {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}}L_{\delta }(a)={\begin{cases}{\frac  {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac  {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}
+$$
 
 <p>
 We will discuss in more

doc/pub/How2ReadData/html/How2ReadData.html

@@ -1168,7 +1168,10 @@ years etc.
 <p>
 Finally, another cost function is the Huber cost function used in robust regression.
 It is less sensitive to outliers in data than the squared error cost function.
-A variant for classification is also sometimes used, a quantity we will meet later.
+A variant for classification is also sometimes used, a quantity we will meet later. 
+$$
+L_{\delta }(a)={\begin{cases}{\frac {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}}L_{\delta }(a)={\begin{cases}{\frac  {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac  {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}
+$$
 
 <p>
 We will discuss in more

doc/pub/How2ReadData/ipynb/How2ReadData.ipynb

@@ -1459,10 +1459,22 @@
     "\n",
     "Finally, another cost function is the Huber cost function used in robust regression.\n",
     "It is less sensitive to outliers in data than the squared error cost function.\n",
-    "A variant for classification is also sometimes used, a quantity we will meet later. \n",
-    "\n",
-    "\n",
-    "\n",
+    "A variant for classification is also sometimes used, a quantity we will meet later."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "$$\n",
+    "L_{\\delta }(a)={\\begin{cases}{\\frac {1}{2}}{a^{2}}&{\\text{for }}|a|\\leq \\delta ,\\\\\\delta (|a|-{\\frac {1}{2}}\\delta ),&{\\text{otherwise.}}\\end{cases}}}L_{\\delta }(a)={\\begin{cases}{\\frac  {1}{2}}{a^{2}}&{\\text{for }}|a|\\leq \\delta ,\\\\\\delta (|a|-{\\frac  {1}{2}}\\delta ),&{\\text{otherwise.}}\\end{cases}}\n",
+    "$$"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
     "We will discuss in more\n",
     "detail these and other functions in the various lectures.  We conclude this part with another example. Instead of \n",
     "a linear $x$-dependence we study now a cubic polynomial and use the polynomial regression analysis tools of scikit-learn."

doc/src/How2ReadData/How2ReadData.do.txt

@@ -940,8 +940,11 @@ years etc.
 Finally, another cost function is the Huber cost function used in robust regression.
 It is less sensitive to outliers in data than the squared error cost function.
 A variant for classification is also sometimes used, a quantity we will meet later. 
-
-
+!bt
+\[
+L_{\delta }(a)={\begin{cases}{\frac {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}}L_{\delta }(a)={\begin{cases}{\frac  {1}{2}}{a^{2}}&{\text{for }}|a|\leq \delta ,\\\delta (|a|-{\frac  {1}{2}}\delta ),&{\text{otherwise.}}\end{cases}}
+\]
+!et
 
 We will discuss in more
 detail these and other functions in the various lectures.  We conclude this part with another example. Instead of