2021-03-06 Archiv

☡.

The dangerous bend or caution symbol ☡ [...] was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.

Wikipedia: Bourbaki dangerous bend symbol