I believe that fedja has disproved a conjecture of Erdős et al. when commenting on my question Almost monochromatic point sets, but I'm unable to contact fedja regarding this. What shall I do? Shall I just casually drop a footnote in my upcoming paper that an unknown user has solved an Erdős problem?
This would not be a first, see arXiv:1909.00177
The citation (which I think follows the best practice) is
 Fedja, Communication on the MathOverflow website (2018), available online at