Why is slow?

Hey guys, i'm trying to run this script but it takes too long. Can i change anything to make it faster?

Note: I've already tried in another computer.

a=1560;                            
b=6360;                    
tol=500;               
it= 50;

g=@(P)(3*10^7*(P^(-0.6)+7/P)+800*P^0.7+400000);

x1=a+(1/4)*((b-a)/4);

x2=a+(2/4)*((b-a)/4);

x3=a+(3/4)*((b-a)/4);

g_x1=g(x1);

g_x2=g(x2);

g_x3=g(x3);

while (abs(b-a)>tol) 

      if g_x1<g_x2
          b=x2;
          x2=x1;
          x1=a+(1/4)*((b-a)/4);

          g_x1=g(x1);
          g_x2=g(x2);
          g_x3=g(x3);

      elseif g_x3<g_x2
              a=x2;
              x2=x3;
              x3=a+(3/4)*((b-a)/4);

              g_x1=g(x1);
              g_x2=g(x2);
              g_x3=g(x3);
      else g_x3>g_x2
              a=x1;
              b=x2;

              g_x1=g(x1);
              g_x2=g(x2);
              g_x3=g(x3);
      end

end

Answer 1

[the cyclist]

The problem is not that your code is slow, but that your algorithm doesn't converge.

abs(b-a) reaches 1925.3906, and stays there, so the while loop doesn't end.

[Eryk Fernandes]

Thank you, but i can't see why it don't converge

[the cyclist]

Do you have a reference for the algorithm you are using? Maybe you coded it incorrectly.

Also, I am quite sure that this line ...

else g_x3>g_x2

is not what you intended. That line is not going to check any condition. It is going to execute the code

g_x3>g_x2

and just display 1 or 0 (i.e. true or false).