a(0) = 4 from the complete graph K_4.
a(1) = 6 from the utility graph K_{3,3}.
a(2) = 10 from the Petersen graph (and 1 other).
a(3) = 14 from the Heawood graph (and 7 others).
a(4) = 16 from the MoebiusKantor graph (and 1 other).
a(5) = 18 from the Pappus graph (and 1 other).
a(6) = 20 from the Desargues graph (and 2 others based on 10_3 configurations).
a(7) = 22 from the 7crossing graphs (4 in total).
a(8) = 24 from the McGee graph (and 3 others).
a(9) = 26 from the McGee graph + edge insertion and edgeexcised Coxeter graph (and unknown others).
a(10) = 28 from the McGee graph + double edge insertion and another from Clancy et al. preprint (and unknown others).
a(11) = 28 from the Coxeter graph (and unknown others).
a(12) <= 30 from the CNG 10A + edge insertion.
a(13) <= 30 from the Levi graph.
a(14) <= 36 from GP(18,5).  Eric W. Weisstein, Apr 15 2019
a(15) <= 40 from GP(20,8).  Eric W. Weisstein, Apr 15 2019
