Probability of an edge in directed random configuration graph












1












$begingroup$


I am considering Bollobas' directed random configuration graph of size $N$, constructed by the following random algorithm:




  1. Draw a sequence of $N$ node-degree pairs $(j_1,k_1),...,(j_N,k_N)$ independently from the degree distribution $P$.
    Accept the draw only if it is feasible, i.e. only if $sum_{n in [N]}(j_n-k_n)=0$.


  2. For every node $n$, create $j_n$ in-stubs and $k_n$ out-stubs, where in-/out-stubs are open ended half-edges with an in-/out-arrow.


  3. For any unpaired out-stub, select iteratively uniformly at random an unpaired in-stub and connect them.


Each such resulting pair of stubs forms a directed edge of the graph.



I am interested in the probabilities of an edge. My suggestion is:



Under this random matching approach, the probability that there is an edge from a node $i$ to a node $l$ is, with $m$ being the number of all edges:
$$p_{il}=frac{k_i cdot j_l}{(2m-1)},$$
as node $i$ has $k_i$ out-stubs and $j_l$ in-stubs out of $2m-1$ (excluding of node $i$) attached to $l$ to which it could connect.



And similarly the probability that there is an edge from a node $l$ to $i$ is:$$p_{il}=frac{j_i cdot k_l}{(2m-1)}$$



Is this correct, or am I missing something?










share|cite|improve this question











$endgroup$

















    1












    $begingroup$


    I am considering Bollobas' directed random configuration graph of size $N$, constructed by the following random algorithm:




    1. Draw a sequence of $N$ node-degree pairs $(j_1,k_1),...,(j_N,k_N)$ independently from the degree distribution $P$.
      Accept the draw only if it is feasible, i.e. only if $sum_{n in [N]}(j_n-k_n)=0$.


    2. For every node $n$, create $j_n$ in-stubs and $k_n$ out-stubs, where in-/out-stubs are open ended half-edges with an in-/out-arrow.


    3. For any unpaired out-stub, select iteratively uniformly at random an unpaired in-stub and connect them.


    Each such resulting pair of stubs forms a directed edge of the graph.



    I am interested in the probabilities of an edge. My suggestion is:



    Under this random matching approach, the probability that there is an edge from a node $i$ to a node $l$ is, with $m$ being the number of all edges:
    $$p_{il}=frac{k_i cdot j_l}{(2m-1)},$$
    as node $i$ has $k_i$ out-stubs and $j_l$ in-stubs out of $2m-1$ (excluding of node $i$) attached to $l$ to which it could connect.



    And similarly the probability that there is an edge from a node $l$ to $i$ is:$$p_{il}=frac{j_i cdot k_l}{(2m-1)}$$



    Is this correct, or am I missing something?










    share|cite|improve this question











    $endgroup$















      1












      1








      1





      $begingroup$


      I am considering Bollobas' directed random configuration graph of size $N$, constructed by the following random algorithm:




      1. Draw a sequence of $N$ node-degree pairs $(j_1,k_1),...,(j_N,k_N)$ independently from the degree distribution $P$.
        Accept the draw only if it is feasible, i.e. only if $sum_{n in [N]}(j_n-k_n)=0$.


      2. For every node $n$, create $j_n$ in-stubs and $k_n$ out-stubs, where in-/out-stubs are open ended half-edges with an in-/out-arrow.


      3. For any unpaired out-stub, select iteratively uniformly at random an unpaired in-stub and connect them.


      Each such resulting pair of stubs forms a directed edge of the graph.



      I am interested in the probabilities of an edge. My suggestion is:



      Under this random matching approach, the probability that there is an edge from a node $i$ to a node $l$ is, with $m$ being the number of all edges:
      $$p_{il}=frac{k_i cdot j_l}{(2m-1)},$$
      as node $i$ has $k_i$ out-stubs and $j_l$ in-stubs out of $2m-1$ (excluding of node $i$) attached to $l$ to which it could connect.



      And similarly the probability that there is an edge from a node $l$ to $i$ is:$$p_{il}=frac{j_i cdot k_l}{(2m-1)}$$



      Is this correct, or am I missing something?










      share|cite|improve this question











      $endgroup$




      I am considering Bollobas' directed random configuration graph of size $N$, constructed by the following random algorithm:




      1. Draw a sequence of $N$ node-degree pairs $(j_1,k_1),...,(j_N,k_N)$ independently from the degree distribution $P$.
        Accept the draw only if it is feasible, i.e. only if $sum_{n in [N]}(j_n-k_n)=0$.


      2. For every node $n$, create $j_n$ in-stubs and $k_n$ out-stubs, where in-/out-stubs are open ended half-edges with an in-/out-arrow.


      3. For any unpaired out-stub, select iteratively uniformly at random an unpaired in-stub and connect them.


      Each such resulting pair of stubs forms a directed edge of the graph.



      I am interested in the probabilities of an edge. My suggestion is:



      Under this random matching approach, the probability that there is an edge from a node $i$ to a node $l$ is, with $m$ being the number of all edges:
      $$p_{il}=frac{k_i cdot j_l}{(2m-1)},$$
      as node $i$ has $k_i$ out-stubs and $j_l$ in-stubs out of $2m-1$ (excluding of node $i$) attached to $l$ to which it could connect.



      And similarly the probability that there is an edge from a node $l$ to $i$ is:$$p_{il}=frac{j_i cdot k_l}{(2m-1)}$$



      Is this correct, or am I missing something?







      probability probability-theory graph-theory random-graphs






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 6 at 19:23







      Alisat

















      asked Nov 9 '18 at 15:09









      AlisatAlisat

      639




      639






















          1 Answer
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          active

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          2












          $begingroup$

          We can choose a uniformly random matching in any order. So let's choose the $k_i$ edges out of node $i$ one at a time and see the probability none of them go to node $l$.



          The first edge we choose has probability $frac{j_l}{m}$ of going to $l$. If it doesn't, then the second edge has probability $frac{j_l}{m-1}$. If that also doesn't happen, then the third edge has probability $frac{j_l}{m-2}$, and so on. So the overall probability that none of the edges go to node $l$ is
          $$
          1-p_{il} = left(1 - frac{j_l}{m}right)left(1 - frac{j_l}{m-1}right) dotsb left(1 - frac{j_l}{m-k_i+1}right)
          $$

          and the probability you want is the complement of this one.



          Asymptotically, however, assuming $k_i$ and $j_l$ are small relative to $m$, the answer is in fact close to $frac{k_i cdot j_l}{m}$ which is what you have, except that $m$ and not $2m-1$ is the number of options for the first edge. (Unlike the undirected configuration model, here there are $m$ out-stubs and $m$ in-stubs, which we distinguish.)



          Additionally, because there are $k_i$ edges with probability $frac{j_l}{m}$ of going to $l$, then $frac{k_i j_l}{m}$ edges is the expected number of edges from $i$ to $l$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
            $endgroup$
            – Alisat
            Jan 6 at 18:35








          • 1




            $begingroup$
            The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:10










          • $begingroup$
            And for the undirected case, the formula you give is also the expectation, not the probability.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:15






          • 1




            $begingroup$
            The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:19








          • 1




            $begingroup$
            Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:48












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          2












          $begingroup$

          We can choose a uniformly random matching in any order. So let's choose the $k_i$ edges out of node $i$ one at a time and see the probability none of them go to node $l$.



          The first edge we choose has probability $frac{j_l}{m}$ of going to $l$. If it doesn't, then the second edge has probability $frac{j_l}{m-1}$. If that also doesn't happen, then the third edge has probability $frac{j_l}{m-2}$, and so on. So the overall probability that none of the edges go to node $l$ is
          $$
          1-p_{il} = left(1 - frac{j_l}{m}right)left(1 - frac{j_l}{m-1}right) dotsb left(1 - frac{j_l}{m-k_i+1}right)
          $$

          and the probability you want is the complement of this one.



          Asymptotically, however, assuming $k_i$ and $j_l$ are small relative to $m$, the answer is in fact close to $frac{k_i cdot j_l}{m}$ which is what you have, except that $m$ and not $2m-1$ is the number of options for the first edge. (Unlike the undirected configuration model, here there are $m$ out-stubs and $m$ in-stubs, which we distinguish.)



          Additionally, because there are $k_i$ edges with probability $frac{j_l}{m}$ of going to $l$, then $frac{k_i j_l}{m}$ edges is the expected number of edges from $i$ to $l$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
            $endgroup$
            – Alisat
            Jan 6 at 18:35








          • 1




            $begingroup$
            The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:10










          • $begingroup$
            And for the undirected case, the formula you give is also the expectation, not the probability.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:15






          • 1




            $begingroup$
            The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:19








          • 1




            $begingroup$
            Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:48
















          2












          $begingroup$

          We can choose a uniformly random matching in any order. So let's choose the $k_i$ edges out of node $i$ one at a time and see the probability none of them go to node $l$.



          The first edge we choose has probability $frac{j_l}{m}$ of going to $l$. If it doesn't, then the second edge has probability $frac{j_l}{m-1}$. If that also doesn't happen, then the third edge has probability $frac{j_l}{m-2}$, and so on. So the overall probability that none of the edges go to node $l$ is
          $$
          1-p_{il} = left(1 - frac{j_l}{m}right)left(1 - frac{j_l}{m-1}right) dotsb left(1 - frac{j_l}{m-k_i+1}right)
          $$

          and the probability you want is the complement of this one.



          Asymptotically, however, assuming $k_i$ and $j_l$ are small relative to $m$, the answer is in fact close to $frac{k_i cdot j_l}{m}$ which is what you have, except that $m$ and not $2m-1$ is the number of options for the first edge. (Unlike the undirected configuration model, here there are $m$ out-stubs and $m$ in-stubs, which we distinguish.)



          Additionally, because there are $k_i$ edges with probability $frac{j_l}{m}$ of going to $l$, then $frac{k_i j_l}{m}$ edges is the expected number of edges from $i$ to $l$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
            $endgroup$
            – Alisat
            Jan 6 at 18:35








          • 1




            $begingroup$
            The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:10










          • $begingroup$
            And for the undirected case, the formula you give is also the expectation, not the probability.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:15






          • 1




            $begingroup$
            The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:19








          • 1




            $begingroup$
            Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:48














          2












          2








          2





          $begingroup$

          We can choose a uniformly random matching in any order. So let's choose the $k_i$ edges out of node $i$ one at a time and see the probability none of them go to node $l$.



          The first edge we choose has probability $frac{j_l}{m}$ of going to $l$. If it doesn't, then the second edge has probability $frac{j_l}{m-1}$. If that also doesn't happen, then the third edge has probability $frac{j_l}{m-2}$, and so on. So the overall probability that none of the edges go to node $l$ is
          $$
          1-p_{il} = left(1 - frac{j_l}{m}right)left(1 - frac{j_l}{m-1}right) dotsb left(1 - frac{j_l}{m-k_i+1}right)
          $$

          and the probability you want is the complement of this one.



          Asymptotically, however, assuming $k_i$ and $j_l$ are small relative to $m$, the answer is in fact close to $frac{k_i cdot j_l}{m}$ which is what you have, except that $m$ and not $2m-1$ is the number of options for the first edge. (Unlike the undirected configuration model, here there are $m$ out-stubs and $m$ in-stubs, which we distinguish.)



          Additionally, because there are $k_i$ edges with probability $frac{j_l}{m}$ of going to $l$, then $frac{k_i j_l}{m}$ edges is the expected number of edges from $i$ to $l$.






          share|cite|improve this answer











          $endgroup$



          We can choose a uniformly random matching in any order. So let's choose the $k_i$ edges out of node $i$ one at a time and see the probability none of them go to node $l$.



          The first edge we choose has probability $frac{j_l}{m}$ of going to $l$. If it doesn't, then the second edge has probability $frac{j_l}{m-1}$. If that also doesn't happen, then the third edge has probability $frac{j_l}{m-2}$, and so on. So the overall probability that none of the edges go to node $l$ is
          $$
          1-p_{il} = left(1 - frac{j_l}{m}right)left(1 - frac{j_l}{m-1}right) dotsb left(1 - frac{j_l}{m-k_i+1}right)
          $$

          and the probability you want is the complement of this one.



          Asymptotically, however, assuming $k_i$ and $j_l$ are small relative to $m$, the answer is in fact close to $frac{k_i cdot j_l}{m}$ which is what you have, except that $m$ and not $2m-1$ is the number of options for the first edge. (Unlike the undirected configuration model, here there are $m$ out-stubs and $m$ in-stubs, which we distinguish.)



          Additionally, because there are $k_i$ edges with probability $frac{j_l}{m}$ of going to $l$, then $frac{k_i j_l}{m}$ edges is the expected number of edges from $i$ to $l$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 6 at 20:09

























          answered Jan 6 at 17:16









          Misha LavrovMisha Lavrov

          48.8k757107




          48.8k757107












          • $begingroup$
            So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
            $endgroup$
            – Alisat
            Jan 6 at 18:35








          • 1




            $begingroup$
            The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:10










          • $begingroup$
            And for the undirected case, the formula you give is also the expectation, not the probability.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:15






          • 1




            $begingroup$
            The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:19








          • 1




            $begingroup$
            Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:48


















          • $begingroup$
            So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
            $endgroup$
            – Alisat
            Jan 6 at 18:35








          • 1




            $begingroup$
            The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:10










          • $begingroup$
            And for the undirected case, the formula you give is also the expectation, not the probability.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 20:15






          • 1




            $begingroup$
            The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:19








          • 1




            $begingroup$
            Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
            $endgroup$
            – Misha Lavrov
            Jan 6 at 21:48
















          $begingroup$
          So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
          $endgroup$
          – Alisat
          Jan 6 at 18:35






          $begingroup$
          So I guess there is no "neat" form for the probability as in the undirected one, where it is of the form $p_{ij} =frac{ k_i k_j} {2m−1}$. ? (Only for the large limits as you stated $p_{il} =frac{ k_i j_l} {m}$)
          $endgroup$
          – Alisat
          Jan 6 at 18:35






          1




          1




          $begingroup$
          The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 20:10




          $begingroup$
          The "neat" form is not the probability but the expected number of edges from $i$ to $l$.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 20:10












          $begingroup$
          And for the undirected case, the formula you give is also the expectation, not the probability.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 20:15




          $begingroup$
          And for the undirected case, the formula you give is also the expectation, not the probability.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 20:15




          1




          1




          $begingroup$
          The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 21:19






          $begingroup$
          The paper does not use the configuration mode. Instead, they define $p_{ij} = frac{k_i k_j}{2m}$ then add an edge $ij$ with that probability. As they mention on p. 3 (second paragraph) the actual degree of vertex $i$ is not guaranteed to be $k_i$ in this model.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 21:19






          1




          1




          $begingroup$
          Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 21:48




          $begingroup$
          Upon further reading, the paper does seem to claim that $frac{k_i k_j}{2m-1}$ is the exact edge probability in the random matching model, which is incorrect.
          $endgroup$
          – Misha Lavrov
          Jan 6 at 21:48


















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