add lecture to index

mkdocs.yml

@@ -8,6 +8,7 @@ nav:
   - Intro: 'index.md'
   - Coordinates: 'coordinates.md'
   - Complex Numbers: '1_complex_numbers.md'
+  - Differential equations 2: '8_differential_equations_2.md'
 
 theme:
   name: material

src/differential_equations_2.md → src/8_differential_equations_2.md

@@ -574,4 +574,4 @@ necessary to work with numerical methods of solution.
         (a) Lf = A(x) \frac{d^2 f}{dx^2}
 
         (b) Lf = \frac{d}{dx} \big{()} A(x) \frac{df}{dx} \big{)}
-    
\ No newline at end of file
+