Description: The induction step for a vertex degree calculation, for example in the Königsberg graph. If the degree of U in the edge set E is P , then adding { X , U } to the edge set, where X =/= U , yields degree P + 1 . (Contributed by Mario Carneiro, 12-Mar-2015) (Revised by Mario Carneiro, 28-Feb-2016) (Revised by AV, 3-Mar-2021)