| Lecture Index | |
| 0.25 | Pythagoras |
| 0.36 | Triangular numbers |
| 1.38 | Guess the next triangular number |
| 1.56 | triangular number definition |
| 2.49 | Gauss story about the sum of first 100 natural numbers |
| 4.15 | problem |
| 4.29 | solution |
| Elementary explanation of triangular numbers and Gauss demonstration for the sum of the first 100 natural numbers. |