A320420 Number of monoids of order n that are not groups.
0, 0, 1, 6, 33, 227, 2235, 31558, 1668992
Offset: 0
This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.