Verify whether the given functions are invertible or not.

0:00-0:15 Question introduction.
Part 1 
0:16-3:44 Verify the given piecewise function is invertible by graph
3:45-4:40 Write the invertible function equation (additional)
Part 2
4:41-8:52 Verify the summation system is not invertible by inputing a discrete delta function