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  2. Handshaking lemma - Wikipedia

    en.wikipedia.org/wiki/Handshaking_lemma

    The sum of degrees of all six vertices is 2 + 3 + 2 + 3 + 3 + 1 = 14, twice the number of edges. In graph theory , the handshaking lemma is the statement that, in every finite undirected graph , the number of vertices that touch an odd number of edges is even.

  3. Internal and external angles - Wikipedia

    en.wikipedia.org/wiki/Internal_and_external_angles

    The sum of all the internal angles of a simple polygon is π(n−2) radians or 180(n–2) degrees, where n is the number of sides. The formula can be proved by using mathematical induction: starting with a triangle, for which the angle sum is 180°, then replacing one side with two sides connected at another vertex, and so on.

  4. Angular defect - Wikipedia

    en.wikipedia.org/wiki/Angular_defect

    For a polyhedron, the defect at a vertex equals 2π minus the sum of all the angles at the vertex (all the faces at the vertex are included). If a polyhedron is convex, then the defect of each vertex is always positive. If the sum of the angles exceeds a full turn, as occurs in some vertices of many non-convex polyhedra, then the defect is ...

  5. Glossary of graph theory - Wikipedia

    en.wikipedia.org/wiki/Glossary_of_graph_theory

    The total degree is the sum of the degrees of all vertices; by the handshaking lemma it is an even number. The degree sequence is the collection of degrees of all vertices, in sorted order from largest to smallest. In a directed graph, one may distinguish the in-degree (number of incoming edges) and out-degree (number of outgoing edges).

  6. Parallelogram - Wikipedia

    en.wikipedia.org/wiki/Parallelogram

    The sum of the squares of the sides equals the sum of the squares of the diagonals. (This is the parallelogram law.) It has rotational symmetry of order 2. The sum of the distances from any interior point to the sides is independent of the location of the point. [4] (This is an extension of Viviani's theorem.)

  7. Arrangement of lines - Wikipedia

    en.wikipedia.org/wiki/Arrangement_of_lines

    This maximum is attained for simple arrangements, those in which each two lines cross at a vertex that is disjoint from all the other lines. The number of vertices is smaller when some lines are parallel, or when some vertices are crossed by more than two lines. [4] An arrangement can be rotated, if necessary, to avoid axis-parallel lines.

  8. Degree (graph theory) - Wikipedia

    en.wikipedia.org/wiki/Degree_(graph_theory)

    The degree sum formula states that, given a graph = (,), ⁡ = | |. The formula implies that in any undirected graph, the number of vertices with odd degree is even. This statement (as well as the degree sum formula) is known as the handshaking lemma. The latter name comes from a popular mathematical problem, which is to prove that in any group ...

  9. Spherical geometry - Wikipedia

    en.wikipedia.org/wiki/Spherical_geometry

    The sum of the angles of a spherical triangle is not equal to 180°. A sphere is a curved surface, but locally the laws of the flat (planar) Euclidean geometry are good approximations. In a small triangle on the face of the earth, the sum of the angles is only slightly more than 180 degrees. A sphere with a spherical triangle on it.