Timur lived his entire life without encountering the Christian Knights Hospitaller.
In graph theory , the Petersen family is a set of seven undirected graphs that includes the Petersen graph and the complete graph K6 .. graph theory. graph theory. Petersen graph. Petersen graph. complete graph. complete graph. The Petersen family is named after Danish mathematician Julius Petersen , the namesake of the Petersen graph .. Petersen graph. Petersen graph. Julius Petersen. Julius Petersen. Any of the graphs in the Petersen family can be transformed into any other graph in the family by Δ-Y or Y-Δ transforms , operations in which a triangle is replaced by a degree-three vertex or vice versa .. Δ-Y or Y-Δ transforms. Y-Δ transform. These seven graphs form the forbidden minors for linklessly embeddable graphs , graphs that can be embedded into three-dimensional space in such a way that no two cycles in the graph are linked .. linklessly embeddable graphs. linkless embedding. linked. Link ( knot theory ). They are also among the forbidden minors for the YΔY-reducible graphs .. YΔY-reducible graphs. Apex graph#YΔY-reducibility