Can someone explain to me the significance of $e leq 3v-6$ in graph theory?












-1












$begingroup$


I'm studying for a final and my textbook often uses the equation
$$
e le 3v-6
$$

(seems to be a theory or corollary) for some of the graph theory proofs, but I can't find anywhere as to where this equation is derived from therefore making it hard for me to attempt to use it as part of a solution.




Could someone please tell me how it's derived and why it's significant?




Much Thanks!










share|cite|improve this question











$endgroup$

















    -1












    $begingroup$


    I'm studying for a final and my textbook often uses the equation
    $$
    e le 3v-6
    $$

    (seems to be a theory or corollary) for some of the graph theory proofs, but I can't find anywhere as to where this equation is derived from therefore making it hard for me to attempt to use it as part of a solution.




    Could someone please tell me how it's derived and why it's significant?




    Much Thanks!










    share|cite|improve this question











    $endgroup$















      -1












      -1








      -1





      $begingroup$


      I'm studying for a final and my textbook often uses the equation
      $$
      e le 3v-6
      $$

      (seems to be a theory or corollary) for some of the graph theory proofs, but I can't find anywhere as to where this equation is derived from therefore making it hard for me to attempt to use it as part of a solution.




      Could someone please tell me how it's derived and why it's significant?




      Much Thanks!










      share|cite|improve this question











      $endgroup$




      I'm studying for a final and my textbook often uses the equation
      $$
      e le 3v-6
      $$

      (seems to be a theory or corollary) for some of the graph theory proofs, but I can't find anywhere as to where this equation is derived from therefore making it hard for me to attempt to use it as part of a solution.




      Could someone please tell me how it's derived and why it's significant?




      Much Thanks!







      combinatorics graph-theory






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 5 '18 at 18:46









      user376343

      3,9234829




      3,9234829










      asked Dec 5 '18 at 18:26









      DevAllanPerDevAllanPer

      1336




      1336






















          5 Answers
          5






          active

          oldest

          votes


















          3












          $begingroup$

          For a simple, connected, planar graph with $v ge 3$ vertices and $e$ edges,
          $e le 3 v - 6$. See Wikipedia.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58



















          2












          $begingroup$

          This is only applicable to planar graphs, for instance $K_5$ has $5$ vertices but $10 > 15 - 6$ edges. This answer will give you a proof to this common theorem.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58










          • $begingroup$
            You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
            $endgroup$
            – Larry B.
            Dec 5 '18 at 20:06



















          1












          $begingroup$

          This formula is valid specifically for planar graphs. If a graph with $3$ or more vertices is planar, and $e$ is the number of edges and $v$ the number of vertices of that graph, then it is true that
          $$
          e leq 3v-6
          $$






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58










          • $begingroup$
            @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
            $endgroup$
            – Arthur
            Dec 5 '18 at 19:56





















          1












          $begingroup$

          Let $G$ be a planar graph and let $F$ denote the number of faces and $E$ denote the number of edges, and $V$ denote the number of vertices. Euler's formula says that $V-E+F=2$ which means $F=2+E-V$.
          On the other hand, the length of perimeter surrounding a face is at least $3$. So then if we sum up the length of each face, which is twice number of edges, we get:
          $6+3E-3V=3(2+E-V)=F*3 leq sum_{f in face(G)}{len(f)}=2*E$. which means $E leq 3V-6$.






          share|cite|improve this answer









          $endgroup$





















            0












            $begingroup$

            That's the number of edges in a planar graph with all faces triangles, and thus the highest possible number of edges in a planar graph.
            begin{align*}V+F &= E+2\
            V+frac23 E &= E+2\
            V-2 &= frac13E\
            3V-6 &= Eend{align*}






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              So does it change if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:48










            • $begingroup$
              Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
              $endgroup$
              – jmerry
              Dec 5 '18 at 19:42











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            5 Answers
            5






            active

            oldest

            votes








            5 Answers
            5






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            3












            $begingroup$

            For a simple, connected, planar graph with $v ge 3$ vertices and $e$ edges,
            $e le 3 v - 6$. See Wikipedia.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58
















            3












            $begingroup$

            For a simple, connected, planar graph with $v ge 3$ vertices and $e$ edges,
            $e le 3 v - 6$. See Wikipedia.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58














            3












            3








            3





            $begingroup$

            For a simple, connected, planar graph with $v ge 3$ vertices and $e$ edges,
            $e le 3 v - 6$. See Wikipedia.






            share|cite|improve this answer









            $endgroup$



            For a simple, connected, planar graph with $v ge 3$ vertices and $e$ edges,
            $e le 3 v - 6$. See Wikipedia.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 5 '18 at 18:34









            Robert IsraelRobert Israel

            326k23215469




            326k23215469












            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58


















            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58
















            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58




            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58











            2












            $begingroup$

            This is only applicable to planar graphs, for instance $K_5$ has $5$ vertices but $10 > 15 - 6$ edges. This answer will give you a proof to this common theorem.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
              $endgroup$
              – Larry B.
              Dec 5 '18 at 20:06
















            2












            $begingroup$

            This is only applicable to planar graphs, for instance $K_5$ has $5$ vertices but $10 > 15 - 6$ edges. This answer will give you a proof to this common theorem.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
              $endgroup$
              – Larry B.
              Dec 5 '18 at 20:06














            2












            2








            2





            $begingroup$

            This is only applicable to planar graphs, for instance $K_5$ has $5$ vertices but $10 > 15 - 6$ edges. This answer will give you a proof to this common theorem.






            share|cite|improve this answer









            $endgroup$



            This is only applicable to planar graphs, for instance $K_5$ has $5$ vertices but $10 > 15 - 6$ edges. This answer will give you a proof to this common theorem.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 5 '18 at 18:34









            Larry B.Larry B.

            2,801828




            2,801828












            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
              $endgroup$
              – Larry B.
              Dec 5 '18 at 20:06


















            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
              $endgroup$
              – Larry B.
              Dec 5 '18 at 20:06
















            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58




            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58












            $begingroup$
            You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
            $endgroup$
            – Larry B.
            Dec 5 '18 at 20:06




            $begingroup$
            You can shape the faces however you want, but $K_5$ will be non-planar no matter how you draw it.
            $endgroup$
            – Larry B.
            Dec 5 '18 at 20:06











            1












            $begingroup$

            This formula is valid specifically for planar graphs. If a graph with $3$ or more vertices is planar, and $e$ is the number of edges and $v$ the number of vertices of that graph, then it is true that
            $$
            e leq 3v-6
            $$






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
              $endgroup$
              – Arthur
              Dec 5 '18 at 19:56


















            1












            $begingroup$

            This formula is valid specifically for planar graphs. If a graph with $3$ or more vertices is planar, and $e$ is the number of edges and $v$ the number of vertices of that graph, then it is true that
            $$
            e leq 3v-6
            $$






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
              $endgroup$
              – Arthur
              Dec 5 '18 at 19:56
















            1












            1








            1





            $begingroup$

            This formula is valid specifically for planar graphs. If a graph with $3$ or more vertices is planar, and $e$ is the number of edges and $v$ the number of vertices of that graph, then it is true that
            $$
            e leq 3v-6
            $$






            share|cite|improve this answer









            $endgroup$



            This formula is valid specifically for planar graphs. If a graph with $3$ or more vertices is planar, and $e$ is the number of edges and $v$ the number of vertices of that graph, then it is true that
            $$
            e leq 3v-6
            $$







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 5 '18 at 18:34









            ArthurArthur

            116k7116199




            116k7116199












            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
              $endgroup$
              – Arthur
              Dec 5 '18 at 19:56




















            • $begingroup$
              does this change at all if the faces were per se, a pentagon?
              $endgroup$
              – DevAllanPer
              Dec 5 '18 at 18:58










            • $begingroup$
              @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
              $endgroup$
              – Arthur
              Dec 5 '18 at 19:56


















            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58




            $begingroup$
            does this change at all if the faces were per se, a pentagon?
            $endgroup$
            – DevAllanPer
            Dec 5 '18 at 18:58












            $begingroup$
            @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
            $endgroup$
            – Arthur
            Dec 5 '18 at 19:56






            $begingroup$
            @DevAllanPer You won't get equality if there are pentagonal faces (note that each pentagonal face can be made into three triangular faces by adding two edges between the vertices). So it could be strengthened. To what? I don't know. Something like (I'm just guessing wildly here) $eleq v + 3$, perhaps? A closer look with Euler's formula will probably reveal an actual answer.
            $endgroup$
            – Arthur
            Dec 5 '18 at 19:56













            1












            $begingroup$

            Let $G$ be a planar graph and let $F$ denote the number of faces and $E$ denote the number of edges, and $V$ denote the number of vertices. Euler's formula says that $V-E+F=2$ which means $F=2+E-V$.
            On the other hand, the length of perimeter surrounding a face is at least $3$. So then if we sum up the length of each face, which is twice number of edges, we get:
            $6+3E-3V=3(2+E-V)=F*3 leq sum_{f in face(G)}{len(f)}=2*E$. which means $E leq 3V-6$.






            share|cite|improve this answer









            $endgroup$


















              1












              $begingroup$

              Let $G$ be a planar graph and let $F$ denote the number of faces and $E$ denote the number of edges, and $V$ denote the number of vertices. Euler's formula says that $V-E+F=2$ which means $F=2+E-V$.
              On the other hand, the length of perimeter surrounding a face is at least $3$. So then if we sum up the length of each face, which is twice number of edges, we get:
              $6+3E-3V=3(2+E-V)=F*3 leq sum_{f in face(G)}{len(f)}=2*E$. which means $E leq 3V-6$.






              share|cite|improve this answer









              $endgroup$
















                1












                1








                1





                $begingroup$

                Let $G$ be a planar graph and let $F$ denote the number of faces and $E$ denote the number of edges, and $V$ denote the number of vertices. Euler's formula says that $V-E+F=2$ which means $F=2+E-V$.
                On the other hand, the length of perimeter surrounding a face is at least $3$. So then if we sum up the length of each face, which is twice number of edges, we get:
                $6+3E-3V=3(2+E-V)=F*3 leq sum_{f in face(G)}{len(f)}=2*E$. which means $E leq 3V-6$.






                share|cite|improve this answer









                $endgroup$



                Let $G$ be a planar graph and let $F$ denote the number of faces and $E$ denote the number of edges, and $V$ denote the number of vertices. Euler's formula says that $V-E+F=2$ which means $F=2+E-V$.
                On the other hand, the length of perimeter surrounding a face is at least $3$. So then if we sum up the length of each face, which is twice number of edges, we get:
                $6+3E-3V=3(2+E-V)=F*3 leq sum_{f in face(G)}{len(f)}=2*E$. which means $E leq 3V-6$.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Dec 5 '18 at 19:21









                nafhgoodnafhgood

                1,803422




                1,803422























                    0












                    $begingroup$

                    That's the number of edges in a planar graph with all faces triangles, and thus the highest possible number of edges in a planar graph.
                    begin{align*}V+F &= E+2\
                    V+frac23 E &= E+2\
                    V-2 &= frac13E\
                    3V-6 &= Eend{align*}






                    share|cite|improve this answer









                    $endgroup$













                    • $begingroup$
                      So does it change if the faces were per se, a pentagon?
                      $endgroup$
                      – DevAllanPer
                      Dec 5 '18 at 18:48










                    • $begingroup$
                      Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                      $endgroup$
                      – jmerry
                      Dec 5 '18 at 19:42
















                    0












                    $begingroup$

                    That's the number of edges in a planar graph with all faces triangles, and thus the highest possible number of edges in a planar graph.
                    begin{align*}V+F &= E+2\
                    V+frac23 E &= E+2\
                    V-2 &= frac13E\
                    3V-6 &= Eend{align*}






                    share|cite|improve this answer









                    $endgroup$













                    • $begingroup$
                      So does it change if the faces were per se, a pentagon?
                      $endgroup$
                      – DevAllanPer
                      Dec 5 '18 at 18:48










                    • $begingroup$
                      Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                      $endgroup$
                      – jmerry
                      Dec 5 '18 at 19:42














                    0












                    0








                    0





                    $begingroup$

                    That's the number of edges in a planar graph with all faces triangles, and thus the highest possible number of edges in a planar graph.
                    begin{align*}V+F &= E+2\
                    V+frac23 E &= E+2\
                    V-2 &= frac13E\
                    3V-6 &= Eend{align*}






                    share|cite|improve this answer









                    $endgroup$



                    That's the number of edges in a planar graph with all faces triangles, and thus the highest possible number of edges in a planar graph.
                    begin{align*}V+F &= E+2\
                    V+frac23 E &= E+2\
                    V-2 &= frac13E\
                    3V-6 &= Eend{align*}







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 5 '18 at 18:35









                    jmerryjmerry

                    12k1628




                    12k1628












                    • $begingroup$
                      So does it change if the faces were per se, a pentagon?
                      $endgroup$
                      – DevAllanPer
                      Dec 5 '18 at 18:48










                    • $begingroup$
                      Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                      $endgroup$
                      – jmerry
                      Dec 5 '18 at 19:42


















                    • $begingroup$
                      So does it change if the faces were per se, a pentagon?
                      $endgroup$
                      – DevAllanPer
                      Dec 5 '18 at 18:48










                    • $begingroup$
                      Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                      $endgroup$
                      – jmerry
                      Dec 5 '18 at 19:42
















                    $begingroup$
                    So does it change if the faces were per se, a pentagon?
                    $endgroup$
                    – DevAllanPer
                    Dec 5 '18 at 18:48




                    $begingroup$
                    So does it change if the faces were per se, a pentagon?
                    $endgroup$
                    – DevAllanPer
                    Dec 5 '18 at 18:48












                    $begingroup$
                    Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                    $endgroup$
                    – jmerry
                    Dec 5 '18 at 19:42




                    $begingroup$
                    Faces with more edges lead to fewer total edges. Of course, we could always draw in new edges in each face, cutting them up until only triangles remain.
                    $endgroup$
                    – jmerry
                    Dec 5 '18 at 19:42


















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