For a polynomial F with dependence on a parameter beta, find the value of beta for which F has 2 positive roots

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Good morning,



I have the following polynomial with degree 4 and its coefficients are quadratic functions of a parameter beta:



FF[z_, β_] := z^4 + z^3 (4.67687 - 0.16 β^2) + z^2 (-0.703991 - 0.652433 β^2) +
z (0.0676982 - 0.049884 β^2) - 0.000608244 β^2 + 0.0000234502;


Consider z > 0 and β > 0. From the code



Manipulate[Plot[FF[z, param], z, 0, 0.2, PlotRange -> All, 
PlotLabel -> FF[z, param]],
Column[z /. NSolve[FF[z, param] == 0, z]], CountRoots[FF[z, param], z, 0, ∞],
param, 0.6, 0.18, 0.8]


I know that for some $beta$ (close to 0.6), the quartic polynomial has exactly 2 positive roots. I'd like to find such a value of beta.



I've tried several things. For instance:



FindRoot[NMinimize[FF[z, β], 0.05 < z < 0.15, z], β, 0.6]


and



NSolve[CountRoots[FF[z, β], z, 0, Infinity] == 2, β, 0.6, 0.8]


I appreciate some suggestions.










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  • With brute force search: $beta =0.60705233167945$
    – Mariusz Iwaniuk
    11 hours ago










  • @user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
    – Alex Pereira
    10 hours ago











  • @mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
    – Alex Pereira
    10 hours ago







  • 1




    This article might be useful.
    – J. M. is computer-less♦
    21 mins ago














up vote
3
down vote

favorite
1












Good morning,



I have the following polynomial with degree 4 and its coefficients are quadratic functions of a parameter beta:



FF[z_, β_] := z^4 + z^3 (4.67687 - 0.16 β^2) + z^2 (-0.703991 - 0.652433 β^2) +
z (0.0676982 - 0.049884 β^2) - 0.000608244 β^2 + 0.0000234502;


Consider z > 0 and β > 0. From the code



Manipulate[Plot[FF[z, param], z, 0, 0.2, PlotRange -> All, 
PlotLabel -> FF[z, param]],
Column[z /. NSolve[FF[z, param] == 0, z]], CountRoots[FF[z, param], z, 0, ∞],
param, 0.6, 0.18, 0.8]


I know that for some $beta$ (close to 0.6), the quartic polynomial has exactly 2 positive roots. I'd like to find such a value of beta.



I've tried several things. For instance:



FindRoot[NMinimize[FF[z, β], 0.05 < z < 0.15, z], β, 0.6]


and



NSolve[CountRoots[FF[z, β], z, 0, Infinity] == 2, β, 0.6, 0.8]


I appreciate some suggestions.










share|improve this question









New contributor




Alex Pereira is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.



















  • With brute force search: $beta =0.60705233167945$
    – Mariusz Iwaniuk
    11 hours ago










  • @user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
    – Alex Pereira
    10 hours ago











  • @mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
    – Alex Pereira
    10 hours ago







  • 1




    This article might be useful.
    – J. M. is computer-less♦
    21 mins ago












up vote
3
down vote

favorite
1









up vote
3
down vote

favorite
1






1





Good morning,



I have the following polynomial with degree 4 and its coefficients are quadratic functions of a parameter beta:



FF[z_, β_] := z^4 + z^3 (4.67687 - 0.16 β^2) + z^2 (-0.703991 - 0.652433 β^2) +
z (0.0676982 - 0.049884 β^2) - 0.000608244 β^2 + 0.0000234502;


Consider z > 0 and β > 0. From the code



Manipulate[Plot[FF[z, param], z, 0, 0.2, PlotRange -> All, 
PlotLabel -> FF[z, param]],
Column[z /. NSolve[FF[z, param] == 0, z]], CountRoots[FF[z, param], z, 0, ∞],
param, 0.6, 0.18, 0.8]


I know that for some $beta$ (close to 0.6), the quartic polynomial has exactly 2 positive roots. I'd like to find such a value of beta.



I've tried several things. For instance:



FindRoot[NMinimize[FF[z, β], 0.05 < z < 0.15, z], β, 0.6]


and



NSolve[CountRoots[FF[z, β], z, 0, Infinity] == 2, β, 0.6, 0.8]


I appreciate some suggestions.










share|improve this question









New contributor




Alex Pereira is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











Good morning,



I have the following polynomial with degree 4 and its coefficients are quadratic functions of a parameter beta:



FF[z_, β_] := z^4 + z^3 (4.67687 - 0.16 β^2) + z^2 (-0.703991 - 0.652433 β^2) +
z (0.0676982 - 0.049884 β^2) - 0.000608244 β^2 + 0.0000234502;


Consider z > 0 and β > 0. From the code



Manipulate[Plot[FF[z, param], z, 0, 0.2, PlotRange -> All, 
PlotLabel -> FF[z, param]],
Column[z /. NSolve[FF[z, param] == 0, z]], CountRoots[FF[z, param], z, 0, ∞],
param, 0.6, 0.18, 0.8]


I know that for some $beta$ (close to 0.6), the quartic polynomial has exactly 2 positive roots. I'd like to find such a value of beta.



I've tried several things. For instance:



FindRoot[NMinimize[FF[z, β], 0.05 < z < 0.15, z], β, 0.6]


and



NSolve[CountRoots[FF[z, β], z, 0, Infinity] == 2, β, 0.6, 0.8]


I appreciate some suggestions.







mathematical-optimization polynomials






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edited 48 mins ago









J. M. is computer-less♦

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95.1k10295454






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asked 13 hours ago









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New contributor





Alex Pereira is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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  • With brute force search: $beta =0.60705233167945$
    – Mariusz Iwaniuk
    11 hours ago










  • @user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
    – Alex Pereira
    10 hours ago











  • @mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
    – Alex Pereira
    10 hours ago







  • 1




    This article might be useful.
    – J. M. is computer-less♦
    21 mins ago
















  • With brute force search: $beta =0.60705233167945$
    – Mariusz Iwaniuk
    11 hours ago










  • @user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
    – Alex Pereira
    10 hours ago











  • @mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
    – Alex Pereira
    10 hours ago







  • 1




    This article might be useful.
    – J. M. is computer-less♦
    21 mins ago















With brute force search: $beta =0.60705233167945$
– Mariusz Iwaniuk
11 hours ago




With brute force search: $beta =0.60705233167945$
– Mariusz Iwaniuk
11 hours ago












@user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
– Alex Pereira
10 hours ago





@user6014 Well, I'm interested about the positive roots. $beta = 0.6$ and $beta=0.601$ provide only one positive solution ;)
– Alex Pereira
10 hours ago













@mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
– Alex Pereira
10 hours ago





@mariusz-iwaniuk, can you show us how to do it? I'm asking this because I'll probably have to consider this problem again but with different numbers on the coefficients of F
– Alex Pereira
10 hours ago





1




1




This article might be useful.
– J. M. is computer-less♦
21 mins ago




This article might be useful.
– J. M. is computer-less♦
21 mins ago










2 Answers
2






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up vote
3
down vote













You can brute force this using a method inspired by binary search.



Your function:



FF[z_, b_] := 
z^4 + z^3 (4.67687 - 0.16 b^2) +
z^2 (-0.703991 - 0.652433 b^2) +
z (0.0676982 - 0.049884 b^2) - 0.000608244 b^2 +
0.0000234502;


Function which cuts the min/max range in half each time, outputting the new min/max to search:



f[x1_, x2_] := 
Module[v1, v2, v3, min = N[x1, 30], max = N[x2, 30], vals,
v1 = NSolve[FF[z, min] == 0, z, Reals];
v2 = NSolve[FF[z, max] == 0, z, Reals];
v3 = NSolve[FF[z, (min + max)/2] == 0, z, Reals];

If[Length@v3 <= 3,
vals = (min + max)/2, max,
vals = min, (min + max)/2
];
vals
]


And using our custom function above with FixedPoint we get a range that is hopefully specific enough for your application (with the floating point numbers in your original equations we're restricted to $MachinePrecision):



In[9]:= FixedPoint[f, .6, .61]
Out[9]= 0.6070523316794493, 0.6070523316794494


So as @Mariusz Iwaniuk said in the comments, you're looking for a number in the ballpark of $0.607052331679449$.



Note that my implementation isn't very general and relies on your specific FF being the format it is. No doubt it could be generalized to fit a broader scope of function definitions, though.



There may be a more elegant math-y way to solve it, but I just resort to computational/numerical methods above.






share|improve this answer





























    up vote
    1
    down vote













    First an extended comment:.



    You could use ContourPlot to get an overview.



     eq=z^4 + z^3 (4.67687 - 0.16 [Beta]^2) +z^2 (-0.703991 - 0.652433 [Beta]^2) +z (0.0676982 - 0.049884 [Beta]^2) -0.000608244 [Beta]^2 +0.0000234502
    ContourPlot[0 ==eq, [Beta], 0, 1, z, -6, 1, MaxRecursion -> 5 ,FrameLabel -> [Beta],z]


    enter image description here



    The solutionplot shows two ranges [Beta]<0.6 and [Beta]>0.78 with only two solutions!



    solution



    The intersting branch of solution is in the range z0<z<z1



    [Beta]0, z0 = 0, z /. Solve[eq == 0 /. [Beta] -> 0, z][[2]] 
    (*0, -0.000345152*)
    [Beta]1, z1 = 1, z /. Solve[eq == 0 /. [Beta] -> 1, z][[4]]
    (*1, 0.271234*)


    Now it's easy to calculate



    beta = [Beta] /. Solve[eq == 0, [Beta]][[2]] (* beta[z]*)

    ParametricPlot[z, beta, z, z0, z1, AxesLabel -> "z", "[Beta]"]


    The two extrema define the beta-values we are looking for



    enter image description here



    beta /. NSolve[D[beta, z] == 0, 0 < z < z1, z]
    (*0.779012, 0.607052*)


    That's it. Hope it helps!






    share|improve this answer






















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      2 Answers
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      2 Answers
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      active

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      active

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      up vote
      3
      down vote













      You can brute force this using a method inspired by binary search.



      Your function:



      FF[z_, b_] := 
      z^4 + z^3 (4.67687 - 0.16 b^2) +
      z^2 (-0.703991 - 0.652433 b^2) +
      z (0.0676982 - 0.049884 b^2) - 0.000608244 b^2 +
      0.0000234502;


      Function which cuts the min/max range in half each time, outputting the new min/max to search:



      f[x1_, x2_] := 
      Module[v1, v2, v3, min = N[x1, 30], max = N[x2, 30], vals,
      v1 = NSolve[FF[z, min] == 0, z, Reals];
      v2 = NSolve[FF[z, max] == 0, z, Reals];
      v3 = NSolve[FF[z, (min + max)/2] == 0, z, Reals];

      If[Length@v3 <= 3,
      vals = (min + max)/2, max,
      vals = min, (min + max)/2
      ];
      vals
      ]


      And using our custom function above with FixedPoint we get a range that is hopefully specific enough for your application (with the floating point numbers in your original equations we're restricted to $MachinePrecision):



      In[9]:= FixedPoint[f, .6, .61]
      Out[9]= 0.6070523316794493, 0.6070523316794494


      So as @Mariusz Iwaniuk said in the comments, you're looking for a number in the ballpark of $0.607052331679449$.



      Note that my implementation isn't very general and relies on your specific FF being the format it is. No doubt it could be generalized to fit a broader scope of function definitions, though.



      There may be a more elegant math-y way to solve it, but I just resort to computational/numerical methods above.






      share|improve this answer


























        up vote
        3
        down vote













        You can brute force this using a method inspired by binary search.



        Your function:



        FF[z_, b_] := 
        z^4 + z^3 (4.67687 - 0.16 b^2) +
        z^2 (-0.703991 - 0.652433 b^2) +
        z (0.0676982 - 0.049884 b^2) - 0.000608244 b^2 +
        0.0000234502;


        Function which cuts the min/max range in half each time, outputting the new min/max to search:



        f[x1_, x2_] := 
        Module[v1, v2, v3, min = N[x1, 30], max = N[x2, 30], vals,
        v1 = NSolve[FF[z, min] == 0, z, Reals];
        v2 = NSolve[FF[z, max] == 0, z, Reals];
        v3 = NSolve[FF[z, (min + max)/2] == 0, z, Reals];

        If[Length@v3 <= 3,
        vals = (min + max)/2, max,
        vals = min, (min + max)/2
        ];
        vals
        ]


        And using our custom function above with FixedPoint we get a range that is hopefully specific enough for your application (with the floating point numbers in your original equations we're restricted to $MachinePrecision):



        In[9]:= FixedPoint[f, .6, .61]
        Out[9]= 0.6070523316794493, 0.6070523316794494


        So as @Mariusz Iwaniuk said in the comments, you're looking for a number in the ballpark of $0.607052331679449$.



        Note that my implementation isn't very general and relies on your specific FF being the format it is. No doubt it could be generalized to fit a broader scope of function definitions, though.



        There may be a more elegant math-y way to solve it, but I just resort to computational/numerical methods above.






        share|improve this answer
























          up vote
          3
          down vote










          up vote
          3
          down vote









          You can brute force this using a method inspired by binary search.



          Your function:



          FF[z_, b_] := 
          z^4 + z^3 (4.67687 - 0.16 b^2) +
          z^2 (-0.703991 - 0.652433 b^2) +
          z (0.0676982 - 0.049884 b^2) - 0.000608244 b^2 +
          0.0000234502;


          Function which cuts the min/max range in half each time, outputting the new min/max to search:



          f[x1_, x2_] := 
          Module[v1, v2, v3, min = N[x1, 30], max = N[x2, 30], vals,
          v1 = NSolve[FF[z, min] == 0, z, Reals];
          v2 = NSolve[FF[z, max] == 0, z, Reals];
          v3 = NSolve[FF[z, (min + max)/2] == 0, z, Reals];

          If[Length@v3 <= 3,
          vals = (min + max)/2, max,
          vals = min, (min + max)/2
          ];
          vals
          ]


          And using our custom function above with FixedPoint we get a range that is hopefully specific enough for your application (with the floating point numbers in your original equations we're restricted to $MachinePrecision):



          In[9]:= FixedPoint[f, .6, .61]
          Out[9]= 0.6070523316794493, 0.6070523316794494


          So as @Mariusz Iwaniuk said in the comments, you're looking for a number in the ballpark of $0.607052331679449$.



          Note that my implementation isn't very general and relies on your specific FF being the format it is. No doubt it could be generalized to fit a broader scope of function definitions, though.



          There may be a more elegant math-y way to solve it, but I just resort to computational/numerical methods above.






          share|improve this answer














          You can brute force this using a method inspired by binary search.



          Your function:



          FF[z_, b_] := 
          z^4 + z^3 (4.67687 - 0.16 b^2) +
          z^2 (-0.703991 - 0.652433 b^2) +
          z (0.0676982 - 0.049884 b^2) - 0.000608244 b^2 +
          0.0000234502;


          Function which cuts the min/max range in half each time, outputting the new min/max to search:



          f[x1_, x2_] := 
          Module[v1, v2, v3, min = N[x1, 30], max = N[x2, 30], vals,
          v1 = NSolve[FF[z, min] == 0, z, Reals];
          v2 = NSolve[FF[z, max] == 0, z, Reals];
          v3 = NSolve[FF[z, (min + max)/2] == 0, z, Reals];

          If[Length@v3 <= 3,
          vals = (min + max)/2, max,
          vals = min, (min + max)/2
          ];
          vals
          ]


          And using our custom function above with FixedPoint we get a range that is hopefully specific enough for your application (with the floating point numbers in your original equations we're restricted to $MachinePrecision):



          In[9]:= FixedPoint[f, .6, .61]
          Out[9]= 0.6070523316794493, 0.6070523316794494


          So as @Mariusz Iwaniuk said in the comments, you're looking for a number in the ballpark of $0.607052331679449$.



          Note that my implementation isn't very general and relies on your specific FF being the format it is. No doubt it could be generalized to fit a broader scope of function definitions, though.



          There may be a more elegant math-y way to solve it, but I just resort to computational/numerical methods above.







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 5 hours ago

























          answered 5 hours ago









          user6014

          2,9261123




          2,9261123




















              up vote
              1
              down vote













              First an extended comment:.



              You could use ContourPlot to get an overview.



               eq=z^4 + z^3 (4.67687 - 0.16 [Beta]^2) +z^2 (-0.703991 - 0.652433 [Beta]^2) +z (0.0676982 - 0.049884 [Beta]^2) -0.000608244 [Beta]^2 +0.0000234502
              ContourPlot[0 ==eq, [Beta], 0, 1, z, -6, 1, MaxRecursion -> 5 ,FrameLabel -> [Beta],z]


              enter image description here



              The solutionplot shows two ranges [Beta]<0.6 and [Beta]>0.78 with only two solutions!



              solution



              The intersting branch of solution is in the range z0<z<z1



              [Beta]0, z0 = 0, z /. Solve[eq == 0 /. [Beta] -> 0, z][[2]] 
              (*0, -0.000345152*)
              [Beta]1, z1 = 1, z /. Solve[eq == 0 /. [Beta] -> 1, z][[4]]
              (*1, 0.271234*)


              Now it's easy to calculate



              beta = [Beta] /. Solve[eq == 0, [Beta]][[2]] (* beta[z]*)

              ParametricPlot[z, beta, z, z0, z1, AxesLabel -> "z", "[Beta]"]


              The two extrema define the beta-values we are looking for



              enter image description here



              beta /. NSolve[D[beta, z] == 0, 0 < z < z1, z]
              (*0.779012, 0.607052*)


              That's it. Hope it helps!






              share|improve this answer


























                up vote
                1
                down vote













                First an extended comment:.



                You could use ContourPlot to get an overview.



                 eq=z^4 + z^3 (4.67687 - 0.16 [Beta]^2) +z^2 (-0.703991 - 0.652433 [Beta]^2) +z (0.0676982 - 0.049884 [Beta]^2) -0.000608244 [Beta]^2 +0.0000234502
                ContourPlot[0 ==eq, [Beta], 0, 1, z, -6, 1, MaxRecursion -> 5 ,FrameLabel -> [Beta],z]


                enter image description here



                The solutionplot shows two ranges [Beta]<0.6 and [Beta]>0.78 with only two solutions!



                solution



                The intersting branch of solution is in the range z0<z<z1



                [Beta]0, z0 = 0, z /. Solve[eq == 0 /. [Beta] -> 0, z][[2]] 
                (*0, -0.000345152*)
                [Beta]1, z1 = 1, z /. Solve[eq == 0 /. [Beta] -> 1, z][[4]]
                (*1, 0.271234*)


                Now it's easy to calculate



                beta = [Beta] /. Solve[eq == 0, [Beta]][[2]] (* beta[z]*)

                ParametricPlot[z, beta, z, z0, z1, AxesLabel -> "z", "[Beta]"]


                The two extrema define the beta-values we are looking for



                enter image description here



                beta /. NSolve[D[beta, z] == 0, 0 < z < z1, z]
                (*0.779012, 0.607052*)


                That's it. Hope it helps!






                share|improve this answer
























                  up vote
                  1
                  down vote










                  up vote
                  1
                  down vote









                  First an extended comment:.



                  You could use ContourPlot to get an overview.



                   eq=z^4 + z^3 (4.67687 - 0.16 [Beta]^2) +z^2 (-0.703991 - 0.652433 [Beta]^2) +z (0.0676982 - 0.049884 [Beta]^2) -0.000608244 [Beta]^2 +0.0000234502
                  ContourPlot[0 ==eq, [Beta], 0, 1, z, -6, 1, MaxRecursion -> 5 ,FrameLabel -> [Beta],z]


                  enter image description here



                  The solutionplot shows two ranges [Beta]<0.6 and [Beta]>0.78 with only two solutions!



                  solution



                  The intersting branch of solution is in the range z0<z<z1



                  [Beta]0, z0 = 0, z /. Solve[eq == 0 /. [Beta] -> 0, z][[2]] 
                  (*0, -0.000345152*)
                  [Beta]1, z1 = 1, z /. Solve[eq == 0 /. [Beta] -> 1, z][[4]]
                  (*1, 0.271234*)


                  Now it's easy to calculate



                  beta = [Beta] /. Solve[eq == 0, [Beta]][[2]] (* beta[z]*)

                  ParametricPlot[z, beta, z, z0, z1, AxesLabel -> "z", "[Beta]"]


                  The two extrema define the beta-values we are looking for



                  enter image description here



                  beta /. NSolve[D[beta, z] == 0, 0 < z < z1, z]
                  (*0.779012, 0.607052*)


                  That's it. Hope it helps!






                  share|improve this answer














                  First an extended comment:.



                  You could use ContourPlot to get an overview.



                   eq=z^4 + z^3 (4.67687 - 0.16 [Beta]^2) +z^2 (-0.703991 - 0.652433 [Beta]^2) +z (0.0676982 - 0.049884 [Beta]^2) -0.000608244 [Beta]^2 +0.0000234502
                  ContourPlot[0 ==eq, [Beta], 0, 1, z, -6, 1, MaxRecursion -> 5 ,FrameLabel -> [Beta],z]


                  enter image description here



                  The solutionplot shows two ranges [Beta]<0.6 and [Beta]>0.78 with only two solutions!



                  solution



                  The intersting branch of solution is in the range z0<z<z1



                  [Beta]0, z0 = 0, z /. Solve[eq == 0 /. [Beta] -> 0, z][[2]] 
                  (*0, -0.000345152*)
                  [Beta]1, z1 = 1, z /. Solve[eq == 0 /. [Beta] -> 1, z][[4]]
                  (*1, 0.271234*)


                  Now it's easy to calculate



                  beta = [Beta] /. Solve[eq == 0, [Beta]][[2]] (* beta[z]*)

                  ParametricPlot[z, beta, z, z0, z1, AxesLabel -> "z", "[Beta]"]


                  The two extrema define the beta-values we are looking for



                  enter image description here



                  beta /. NSolve[D[beta, z] == 0, 0 < z < z1, z]
                  (*0.779012, 0.607052*)


                  That's it. Hope it helps!







                  share|improve this answer














                  share|improve this answer



                  share|improve this answer








                  edited 7 mins ago

























                  answered 1 hour ago









                  Ulrich Neumann

                  5,230413




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