How the two non null-homotopic equivalence classes generate the null-homotopic loop on the torus












1














I am new in Alebraic Topology. Given the torus, we say that the fundamental group of the torus is generated by two loops (or more exactly two equivalent classes of loops). One writes $pi=mathbb{Z}timesmathbb{Z}.$



I don't understand, how the null-homotopic loop, which is the constant loop, is generated by the two generators mentioned above. Can somebody provide an explanation? More even so, I don't see how it functions visually, since the two generators are not null-homotopic.
More precisely, given a null-homotopic loop on the surface on a base point $x$, how this loop will be generated by the two generators mentioned above?










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  • 4




    The identity is in every subgroup.
    – Lord Shark the Unknown
    4 hours ago










  • Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
    – user249018
    4 hours ago










  • What do you think the word "generate" means?
    – Eric Wofsey
    4 hours ago
















1














I am new in Alebraic Topology. Given the torus, we say that the fundamental group of the torus is generated by two loops (or more exactly two equivalent classes of loops). One writes $pi=mathbb{Z}timesmathbb{Z}.$



I don't understand, how the null-homotopic loop, which is the constant loop, is generated by the two generators mentioned above. Can somebody provide an explanation? More even so, I don't see how it functions visually, since the two generators are not null-homotopic.
More precisely, given a null-homotopic loop on the surface on a base point $x$, how this loop will be generated by the two generators mentioned above?










share|cite|improve this question




















  • 4




    The identity is in every subgroup.
    – Lord Shark the Unknown
    4 hours ago










  • Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
    – user249018
    4 hours ago










  • What do you think the word "generate" means?
    – Eric Wofsey
    4 hours ago














1












1








1







I am new in Alebraic Topology. Given the torus, we say that the fundamental group of the torus is generated by two loops (or more exactly two equivalent classes of loops). One writes $pi=mathbb{Z}timesmathbb{Z}.$



I don't understand, how the null-homotopic loop, which is the constant loop, is generated by the two generators mentioned above. Can somebody provide an explanation? More even so, I don't see how it functions visually, since the two generators are not null-homotopic.
More precisely, given a null-homotopic loop on the surface on a base point $x$, how this loop will be generated by the two generators mentioned above?










share|cite|improve this question















I am new in Alebraic Topology. Given the torus, we say that the fundamental group of the torus is generated by two loops (or more exactly two equivalent classes of loops). One writes $pi=mathbb{Z}timesmathbb{Z}.$



I don't understand, how the null-homotopic loop, which is the constant loop, is generated by the two generators mentioned above. Can somebody provide an explanation? More even so, I don't see how it functions visually, since the two generators are not null-homotopic.
More precisely, given a null-homotopic loop on the surface on a base point $x$, how this loop will be generated by the two generators mentioned above?







group-theory algebraic-topology fundamental-groups






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share|cite|improve this question













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edited 4 hours ago









Eric Wofsey

179k12204331




179k12204331










asked 4 hours ago









user249018

371127




371127








  • 4




    The identity is in every subgroup.
    – Lord Shark the Unknown
    4 hours ago










  • Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
    – user249018
    4 hours ago










  • What do you think the word "generate" means?
    – Eric Wofsey
    4 hours ago














  • 4




    The identity is in every subgroup.
    – Lord Shark the Unknown
    4 hours ago










  • Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
    – user249018
    4 hours ago










  • What do you think the word "generate" means?
    – Eric Wofsey
    4 hours ago








4




4




The identity is in every subgroup.
– Lord Shark the Unknown
4 hours ago




The identity is in every subgroup.
– Lord Shark the Unknown
4 hours ago












Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
– user249018
4 hours ago




Thanks. But how it is generated in the case of a null-homotopic loop ? In the case the loops are traversing the torus around the hole or through the hole, one can say for instance that the product of one loop and its inverse is the constant loop, that is the identity. But I dont see how it works in the case a loop is null-homotopic, since the generators are non null-homotopic.
– user249018
4 hours ago












What do you think the word "generate" means?
– Eric Wofsey
4 hours ago




What do you think the word "generate" means?
– Eric Wofsey
4 hours ago










2 Answers
2






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oldest

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1














Let $mathbb{T}^2$ denote the torus and choose a basepoint $p in mathbb{T}^2$. Then we know that $pi_1left(mathbb{T}^2, p right) cong mathbb{Z} times mathbb{Z}$.



Now I think the reason for your confusion is an algebraic one.



Recall that $mathbb{Z} times mathbb{Z}$ has two generators, $a= (1, 0)$ and $b =(0, 1)$. Choose an isomorphism $psi : pi_1left(mathbb{T}^2, p right) to mathbb{Z} times mathbb{Z}$, by surjectivity there exists path classes, $[f], [g] in pi_1left(mathbb{T}^2, p right)$ such that $psi([f]) = a$ and $psi([g]) =b$. Then since $psi$ is an isomorphism we have $[f]$ and $[g]$ to be the two generators of $pi_1left(mathbb{T}^2, p right)$.



Now your question is how the path class of the constant loop $c_p : I to mathbb{T}^2$ defined by $c_p(x) = p$ for all $x in I$, that being $[c_p] in pi_1left(mathbb{T}^2, p right)$ is generated by $[f]$ and $[g]$. Well the answer to that is simple: note that $$[c_p] = 1_{pi_1left(mathbb{T}^2, p right)}$$
that is $[c_p]$ is the identity element of $pi_1left(mathbb{T}^2, p right)$. Then recall the following definition that we have for exponents in groups.




Definition: In any group $(G, cdot)$ for any $x in G$ we define $x^0 = 1_G$ where $1_G$ is the identity element of the group $(G, cdot)$.




Hence since $[f], [g] in pi_1left(mathbb{T}^2, p right)$ and $pi_1left(mathbb{T}^2, p right)$ is indeed a group, we have $$[f]^0 = [g]^0 = 1_{pi_1left(mathbb{T}^2, p right)}.$$



Then we have $$left[c_pright] = [f]^0 * [g]^0$$ and so the constant path at $p$ is indeed generated by the two generators of $pi_1left(mathbb{T}^2, p right)$. And since $[c_p]$ is a nullhomotopic loop, since it is a constant loop by definition, the above shows how a product of two non null-homotopic loops yield a null-homotopic loop.





Note that above even though I've gone into quite a bit of detail, the only real fact I'm using is the following algebraic one. If we have a group $G$ and we have $G = langle A rangle$ for some subset $A subseteq G$ then every element $x in G$ can be written as $x = g_1 dots g_n cdot h_1^{-1} dots h_m^{-1}$ where $g_i, h_i in G$. In particular if we have $G = langle c , d rangle$, that is $G$ is generated by the two elements $c$ and $d$ then we can express $1_G$ as $1_G = c^0 cdot d^0$.






share|cite|improve this answer































    4














    Your confusion seems to be about the meaning of the word "generate". By definition, if $G$ is a group and $Ssubseteq G$, then the subgroup generated by $S$ is the smallest subgroup that contains $S$. Since a subgroup always contains the identity element, any subset of $G$ (even the empty set!) "generates" the identity element.






    share|cite|improve this answer





















    • Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
      – user249018
      3 hours ago








    • 1




      OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
      – Eric Wofsey
      3 hours ago






    • 1




      In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
      – Eric Wofsey
      3 hours ago










    • Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
      – user249018
      3 hours ago












    • I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
      – Eric Wofsey
      3 hours ago













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    2 Answers
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    Let $mathbb{T}^2$ denote the torus and choose a basepoint $p in mathbb{T}^2$. Then we know that $pi_1left(mathbb{T}^2, p right) cong mathbb{Z} times mathbb{Z}$.



    Now I think the reason for your confusion is an algebraic one.



    Recall that $mathbb{Z} times mathbb{Z}$ has two generators, $a= (1, 0)$ and $b =(0, 1)$. Choose an isomorphism $psi : pi_1left(mathbb{T}^2, p right) to mathbb{Z} times mathbb{Z}$, by surjectivity there exists path classes, $[f], [g] in pi_1left(mathbb{T}^2, p right)$ such that $psi([f]) = a$ and $psi([g]) =b$. Then since $psi$ is an isomorphism we have $[f]$ and $[g]$ to be the two generators of $pi_1left(mathbb{T}^2, p right)$.



    Now your question is how the path class of the constant loop $c_p : I to mathbb{T}^2$ defined by $c_p(x) = p$ for all $x in I$, that being $[c_p] in pi_1left(mathbb{T}^2, p right)$ is generated by $[f]$ and $[g]$. Well the answer to that is simple: note that $$[c_p] = 1_{pi_1left(mathbb{T}^2, p right)}$$
    that is $[c_p]$ is the identity element of $pi_1left(mathbb{T}^2, p right)$. Then recall the following definition that we have for exponents in groups.




    Definition: In any group $(G, cdot)$ for any $x in G$ we define $x^0 = 1_G$ where $1_G$ is the identity element of the group $(G, cdot)$.




    Hence since $[f], [g] in pi_1left(mathbb{T}^2, p right)$ and $pi_1left(mathbb{T}^2, p right)$ is indeed a group, we have $$[f]^0 = [g]^0 = 1_{pi_1left(mathbb{T}^2, p right)}.$$



    Then we have $$left[c_pright] = [f]^0 * [g]^0$$ and so the constant path at $p$ is indeed generated by the two generators of $pi_1left(mathbb{T}^2, p right)$. And since $[c_p]$ is a nullhomotopic loop, since it is a constant loop by definition, the above shows how a product of two non null-homotopic loops yield a null-homotopic loop.





    Note that above even though I've gone into quite a bit of detail, the only real fact I'm using is the following algebraic one. If we have a group $G$ and we have $G = langle A rangle$ for some subset $A subseteq G$ then every element $x in G$ can be written as $x = g_1 dots g_n cdot h_1^{-1} dots h_m^{-1}$ where $g_i, h_i in G$. In particular if we have $G = langle c , d rangle$, that is $G$ is generated by the two elements $c$ and $d$ then we can express $1_G$ as $1_G = c^0 cdot d^0$.






    share|cite|improve this answer




























      1














      Let $mathbb{T}^2$ denote the torus and choose a basepoint $p in mathbb{T}^2$. Then we know that $pi_1left(mathbb{T}^2, p right) cong mathbb{Z} times mathbb{Z}$.



      Now I think the reason for your confusion is an algebraic one.



      Recall that $mathbb{Z} times mathbb{Z}$ has two generators, $a= (1, 0)$ and $b =(0, 1)$. Choose an isomorphism $psi : pi_1left(mathbb{T}^2, p right) to mathbb{Z} times mathbb{Z}$, by surjectivity there exists path classes, $[f], [g] in pi_1left(mathbb{T}^2, p right)$ such that $psi([f]) = a$ and $psi([g]) =b$. Then since $psi$ is an isomorphism we have $[f]$ and $[g]$ to be the two generators of $pi_1left(mathbb{T}^2, p right)$.



      Now your question is how the path class of the constant loop $c_p : I to mathbb{T}^2$ defined by $c_p(x) = p$ for all $x in I$, that being $[c_p] in pi_1left(mathbb{T}^2, p right)$ is generated by $[f]$ and $[g]$. Well the answer to that is simple: note that $$[c_p] = 1_{pi_1left(mathbb{T}^2, p right)}$$
      that is $[c_p]$ is the identity element of $pi_1left(mathbb{T}^2, p right)$. Then recall the following definition that we have for exponents in groups.




      Definition: In any group $(G, cdot)$ for any $x in G$ we define $x^0 = 1_G$ where $1_G$ is the identity element of the group $(G, cdot)$.




      Hence since $[f], [g] in pi_1left(mathbb{T}^2, p right)$ and $pi_1left(mathbb{T}^2, p right)$ is indeed a group, we have $$[f]^0 = [g]^0 = 1_{pi_1left(mathbb{T}^2, p right)}.$$



      Then we have $$left[c_pright] = [f]^0 * [g]^0$$ and so the constant path at $p$ is indeed generated by the two generators of $pi_1left(mathbb{T}^2, p right)$. And since $[c_p]$ is a nullhomotopic loop, since it is a constant loop by definition, the above shows how a product of two non null-homotopic loops yield a null-homotopic loop.





      Note that above even though I've gone into quite a bit of detail, the only real fact I'm using is the following algebraic one. If we have a group $G$ and we have $G = langle A rangle$ for some subset $A subseteq G$ then every element $x in G$ can be written as $x = g_1 dots g_n cdot h_1^{-1} dots h_m^{-1}$ where $g_i, h_i in G$. In particular if we have $G = langle c , d rangle$, that is $G$ is generated by the two elements $c$ and $d$ then we can express $1_G$ as $1_G = c^0 cdot d^0$.






      share|cite|improve this answer


























        1












        1








        1






        Let $mathbb{T}^2$ denote the torus and choose a basepoint $p in mathbb{T}^2$. Then we know that $pi_1left(mathbb{T}^2, p right) cong mathbb{Z} times mathbb{Z}$.



        Now I think the reason for your confusion is an algebraic one.



        Recall that $mathbb{Z} times mathbb{Z}$ has two generators, $a= (1, 0)$ and $b =(0, 1)$. Choose an isomorphism $psi : pi_1left(mathbb{T}^2, p right) to mathbb{Z} times mathbb{Z}$, by surjectivity there exists path classes, $[f], [g] in pi_1left(mathbb{T}^2, p right)$ such that $psi([f]) = a$ and $psi([g]) =b$. Then since $psi$ is an isomorphism we have $[f]$ and $[g]$ to be the two generators of $pi_1left(mathbb{T}^2, p right)$.



        Now your question is how the path class of the constant loop $c_p : I to mathbb{T}^2$ defined by $c_p(x) = p$ for all $x in I$, that being $[c_p] in pi_1left(mathbb{T}^2, p right)$ is generated by $[f]$ and $[g]$. Well the answer to that is simple: note that $$[c_p] = 1_{pi_1left(mathbb{T}^2, p right)}$$
        that is $[c_p]$ is the identity element of $pi_1left(mathbb{T}^2, p right)$. Then recall the following definition that we have for exponents in groups.




        Definition: In any group $(G, cdot)$ for any $x in G$ we define $x^0 = 1_G$ where $1_G$ is the identity element of the group $(G, cdot)$.




        Hence since $[f], [g] in pi_1left(mathbb{T}^2, p right)$ and $pi_1left(mathbb{T}^2, p right)$ is indeed a group, we have $$[f]^0 = [g]^0 = 1_{pi_1left(mathbb{T}^2, p right)}.$$



        Then we have $$left[c_pright] = [f]^0 * [g]^0$$ and so the constant path at $p$ is indeed generated by the two generators of $pi_1left(mathbb{T}^2, p right)$. And since $[c_p]$ is a nullhomotopic loop, since it is a constant loop by definition, the above shows how a product of two non null-homotopic loops yield a null-homotopic loop.





        Note that above even though I've gone into quite a bit of detail, the only real fact I'm using is the following algebraic one. If we have a group $G$ and we have $G = langle A rangle$ for some subset $A subseteq G$ then every element $x in G$ can be written as $x = g_1 dots g_n cdot h_1^{-1} dots h_m^{-1}$ where $g_i, h_i in G$. In particular if we have $G = langle c , d rangle$, that is $G$ is generated by the two elements $c$ and $d$ then we can express $1_G$ as $1_G = c^0 cdot d^0$.






        share|cite|improve this answer














        Let $mathbb{T}^2$ denote the torus and choose a basepoint $p in mathbb{T}^2$. Then we know that $pi_1left(mathbb{T}^2, p right) cong mathbb{Z} times mathbb{Z}$.



        Now I think the reason for your confusion is an algebraic one.



        Recall that $mathbb{Z} times mathbb{Z}$ has two generators, $a= (1, 0)$ and $b =(0, 1)$. Choose an isomorphism $psi : pi_1left(mathbb{T}^2, p right) to mathbb{Z} times mathbb{Z}$, by surjectivity there exists path classes, $[f], [g] in pi_1left(mathbb{T}^2, p right)$ such that $psi([f]) = a$ and $psi([g]) =b$. Then since $psi$ is an isomorphism we have $[f]$ and $[g]$ to be the two generators of $pi_1left(mathbb{T}^2, p right)$.



        Now your question is how the path class of the constant loop $c_p : I to mathbb{T}^2$ defined by $c_p(x) = p$ for all $x in I$, that being $[c_p] in pi_1left(mathbb{T}^2, p right)$ is generated by $[f]$ and $[g]$. Well the answer to that is simple: note that $$[c_p] = 1_{pi_1left(mathbb{T}^2, p right)}$$
        that is $[c_p]$ is the identity element of $pi_1left(mathbb{T}^2, p right)$. Then recall the following definition that we have for exponents in groups.




        Definition: In any group $(G, cdot)$ for any $x in G$ we define $x^0 = 1_G$ where $1_G$ is the identity element of the group $(G, cdot)$.




        Hence since $[f], [g] in pi_1left(mathbb{T}^2, p right)$ and $pi_1left(mathbb{T}^2, p right)$ is indeed a group, we have $$[f]^0 = [g]^0 = 1_{pi_1left(mathbb{T}^2, p right)}.$$



        Then we have $$left[c_pright] = [f]^0 * [g]^0$$ and so the constant path at $p$ is indeed generated by the two generators of $pi_1left(mathbb{T}^2, p right)$. And since $[c_p]$ is a nullhomotopic loop, since it is a constant loop by definition, the above shows how a product of two non null-homotopic loops yield a null-homotopic loop.





        Note that above even though I've gone into quite a bit of detail, the only real fact I'm using is the following algebraic one. If we have a group $G$ and we have $G = langle A rangle$ for some subset $A subseteq G$ then every element $x in G$ can be written as $x = g_1 dots g_n cdot h_1^{-1} dots h_m^{-1}$ where $g_i, h_i in G$. In particular if we have $G = langle c , d rangle$, that is $G$ is generated by the two elements $c$ and $d$ then we can express $1_G$ as $1_G = c^0 cdot d^0$.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited 1 hour ago

























        answered 1 hour ago









        Perturbative

        4,07011449




        4,07011449























            4














            Your confusion seems to be about the meaning of the word "generate". By definition, if $G$ is a group and $Ssubseteq G$, then the subgroup generated by $S$ is the smallest subgroup that contains $S$. Since a subgroup always contains the identity element, any subset of $G$ (even the empty set!) "generates" the identity element.






            share|cite|improve this answer





















            • Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
              – user249018
              3 hours ago








            • 1




              OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
              – Eric Wofsey
              3 hours ago






            • 1




              In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
              – Eric Wofsey
              3 hours ago










            • Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
              – user249018
              3 hours ago












            • I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
              – Eric Wofsey
              3 hours ago


















            4














            Your confusion seems to be about the meaning of the word "generate". By definition, if $G$ is a group and $Ssubseteq G$, then the subgroup generated by $S$ is the smallest subgroup that contains $S$. Since a subgroup always contains the identity element, any subset of $G$ (even the empty set!) "generates" the identity element.






            share|cite|improve this answer





















            • Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
              – user249018
              3 hours ago








            • 1




              OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
              – Eric Wofsey
              3 hours ago






            • 1




              In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
              – Eric Wofsey
              3 hours ago










            • Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
              – user249018
              3 hours ago












            • I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
              – Eric Wofsey
              3 hours ago
















            4












            4








            4






            Your confusion seems to be about the meaning of the word "generate". By definition, if $G$ is a group and $Ssubseteq G$, then the subgroup generated by $S$ is the smallest subgroup that contains $S$. Since a subgroup always contains the identity element, any subset of $G$ (even the empty set!) "generates" the identity element.






            share|cite|improve this answer












            Your confusion seems to be about the meaning of the word "generate". By definition, if $G$ is a group and $Ssubseteq G$, then the subgroup generated by $S$ is the smallest subgroup that contains $S$. Since a subgroup always contains the identity element, any subset of $G$ (even the empty set!) "generates" the identity element.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered 4 hours ago









            Eric Wofsey

            179k12204331




            179k12204331












            • Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
              – user249018
              3 hours ago








            • 1




              OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
              – Eric Wofsey
              3 hours ago






            • 1




              In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
              – Eric Wofsey
              3 hours ago










            • Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
              – user249018
              3 hours ago












            • I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
              – Eric Wofsey
              3 hours ago




















            • Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
              – user249018
              3 hours ago








            • 1




              OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
              – Eric Wofsey
              3 hours ago






            • 1




              In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
              – Eric Wofsey
              3 hours ago










            • Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
              – user249018
              3 hours ago












            • I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
              – Eric Wofsey
              3 hours ago


















            Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
            – user249018
            3 hours ago






            Thanks. By ''generate'' I meant the group operation on the set of generators. In a group generated by the subset $S$, each element can be written in terms of the generators, which is, elements of $S$. So given the null-homotopic loop on the torus, how can it be put in relation to the elements of $S$, which in our case consists of 2 elements ?
            – user249018
            3 hours ago






            1




            1




            OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
            – Eric Wofsey
            3 hours ago




            OK, but the group multiplication is not the only operation in a group! There are two other operations: the identity element and inverses.
            – Eric Wofsey
            3 hours ago




            1




            1




            In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
            – Eric Wofsey
            3 hours ago




            In particular, one of the operations of a group is an operation which takes no inputs and outputs the identity element. That's how any set "generates" the identity element.
            – Eric Wofsey
            3 hours ago












            Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
            – user249018
            3 hours ago






            Given $Ssubset G$, more precisely the group generated by $S$ is defined as $<S>=SS^{-1}$. So you are right about inverses. The thing with the identity element is less obvious. One excepts it very probably by definition...But when it comes to the fundamental group of the torus, are you saying that the null-homotopic loop is generated by the empty set ? Or maybe we can say the following: the multiplication of a geneartor and its inverse gives us the constant loop, which itself is homotopic to null-homotopic loops. Thus we generate the null-homotopic loop from each one of the two generators ?
            – user249018
            3 hours ago














            I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
            – Eric Wofsey
            3 hours ago






            I don't know what your notation $SS^{-1}$ is supposed to mean, but it sounds like your definition of "the group generated by $S$" is just wrong (which may not be your fault; you may have been taught a wrong definition!). The correct definition is the one I stated in the answer. An equivalent definition is that the subgroup generated by $S$ is the set of all elements of $G$ that can be obtained by starting with elements of $S$ and repeatedly applying the three operations of the group multiplication, inverses, and the identity element.
            – Eric Wofsey
            3 hours ago




















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