Bourbaki dangerous bend symbol
MATHEMATICAL SYMBOL INDICATING PASSAGES TRICKY ON A FIRST READING OR WITH AN ESPECIALLY DIFFICULT ARGUMENT
Dangerous bend symbol; Dangerous bend sign; Bourbaki dangerous bend sign; ☡; Knuth dangerous bend symbol
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.