[orth_proj] [smoothing] write links implicitly (#71)

Author: shlff

lectures/orth_proj.md

@@ -329,7 +329,7 @@ $$
 
 Evidently  $Py$ is a linear function from $y \in \mathbb R^n$ to $P y \in \mathbb R^n$.
 
-This reference is useful [https://en.wikipedia.org/wiki/Linear_map#Matrices](https://en.wikipedia.org/wiki/Linear_map#Matrices).
+[This reference](https://en.wikipedia.org/wiki/Linear_map#Matrices) is useful.
 
 **Theorem.** Let the columns of $n \times k$ matrix $X$ form a basis of $S$.  Then
 
@@ -340,7 +340,7 @@ $$
 Proof: Given arbitrary $y \in \mathbb R^n$ and $P = X (X'X)^{-1} X'$, our claim is that
 
 1. $P y \in S$, and
-1. $y - P y \perp S$
+2. $y - P y \perp S$
 
 Claim 1 is true because
 

lectures/smoothing.md

@@ -115,7 +115,7 @@ payoffs depend on next period's realization of the Markov state.
 * In an $N$ state Markov state version,  $N$ such securities are traded each period.
 * In a continuous state Markov state version, a continuum of such securities are traded each period.
 
-These state-contingent securities are commonly called Arrow securities, after Kenneth Arrow <https://en.wikipedia.org/wiki/Kenneth_Arrow>
+These state-contingent securities are commonly called Arrow securities, after [Kenneth Arrow](https://en.wikipedia.org/wiki/Kenneth_Arrow).
 
 In the **incomplete markets version**, the consumer can buy and sell only one security each period, a risk-free one-period bond with gross
 one-period return $\beta^{-1}$.