add index

src/8_differential_equations_2.md

@@ -6,9 +6,9 @@ title: Differential Equations 2
 
 The lecture on differential equations consists of three parts, each with their own video:
 
-- [Higher order linear differential equations]()
-- [Partial differential equations: Separation of variables]()
-- [Self-adjoint differential operators](#solving-homogeneous-linear-system-with-constant-coefficients)
+- [Higher order linear differential equations](#higher-order-linear-differential-equations)
+- [Partial differential equations: Separation of variables](#partial-differential-equations-separation-of-variables)
+- [Self-adjoint differential operators](#self-adjoint-differential-operators)
 
 **Total video length:  hour  minutes  seconds**