Write to [email protected] for anything at all: a correction, a question about a step, a request for a hint that stops short of the answer, or a problem you think belongs here.
The address is a forwarding alias, so replies arrive from the same mailbox and nothing is filtered into a marketing folder.
Reporting a correction
The fastest useful report contains three things: the puzzle title, the step number where the argument stops working, and one sentence about what happens instead. Counterexamples are welcome and are published with credit if you want the credit.
Response time
Corrections are handled first and usually within a few days. General questions take longer, and questions that ask for a solution to a problem that is currently on the board get a hint rather than an answer.
Advertising and rights
Advertising is handled by Google; there is no direct sales contact and no sponsored entry on this site. For reuse of a proof or a figure, write to the same address and say where it will appear.