“…Notice that two non-isomorphic solutions have the same set of vertex labels. There is one solution for k = 18, (1,8,4,2,5,10), and six for k = 19, namely (1,6,11,3,7,8), (1,7,3,12,5,8), (1,8,7,3,5,12), (1,8,9,4,3,11), (2,7,11,3,4,9), and (3, 4, 5, 6, 11, 7).…”