Abstract
A new recurrence for the number of subgroups of given index in the modular group is derived. The proof requires the derivation of a recurrence for a sequence anbn from recurrences for the an and bn. We show that this is always possible if the an and bn satisfy polynomial recurrences. We also include a short proof of a result of W. W. Stothers on the parity of the number of subgroups of given index in the modular group.
| Original language | English |
|---|---|
| Pages (from-to) | 273-280 |
| Number of pages | 8 |
| Journal | Monatshefte für Mathematik |
| Volume | 87.1979 |
| Issue number | 4 |
| DOIs | |
| Publication status | Published - Dec 1979 |