| Rating | Interpretation |
|---|---|
| 00 | An extremely easy exercise that can be answered immediately if the material of the text has been understood |
| 10 | A simple problem that makes you think over the material just read, but is by no means difficult (max 1 minute) |
| 20 | An average problem that tests basic undestanding of the text material (up to 20 minutes) |
| 30 | A problem of moderate difficulty and/or complexity (up to several hours) |
| 40 | Quite a difficult or lengthy problem that would be suitable for a term project in classroom situations |
| 50 | A research problem that has not yet been solved satisfactorily |
| The remainder of the rating divided by 5 indicates the amount of detailed work required. All problems rated 46 or more are open research problems. |
The arrowhead symbol (”‣”) indicates problems that are of especial value. Readers are not expected to work through all the exercises, but should not view a given solution before giving that exercise a good attempt.
An M symbol indicates a mathematically oriented exercise; a HM symbol indicates that an exercise requires “higher math” such as calculus.