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I have the function Vt

Vn = (-G*Mn)/Sqrt[x^2 + y^2 + z^2 + cn^2];
Vd = (-G*Md)/Sqrt[x^2 + y^2 + (s + Sqrt[h^2 + z^2])^2];
Vb = (G*Mb)/(2*a)*(ArcSinh[(x - a)*(y^2 + z^2 + c^2)^(-1/2)] - 
 ArcSinh[(x + a)*(y^2 + z^2 + c^2)^(-1/2)]);
Vh = (-G*Mh)/Sqrt[x^2 + y^2 + z^2 + ch^2];
Vrot = -(Ωb^2/2)*(x^2 + y^2);

Vt = Vn + Vd + Vb + Vh + Vrot;

G = 1; Mn = 400; cn = 0.25;
Md = 7000; s = 3; h = 0.175;
Mb = 3500; a = 10; c = 1;
Mh = 20000; ch = 20;
Ωb = 4.5;

and I want to expand it in a series around P(x0,0,0), where x0 = 10.63695596. The output should be of the form

V = V(x0,0,0) - A (x - x0)^2 /2 + B y^2 / 2 + C z^2 /2

where V(x0,0,0) = -3242.772174938595.

Any ideas how to get this? It must be very simple.

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  • $\begingroup$ Look up Series. $\endgroup$
    – march
    Nov 20, 2015 at 16:55
  • $\begingroup$ @march I looked the documentation but it does not contain any relevant examp0le on expansion around a 3D point. $\endgroup$
    – Vaggelis_Z
    Nov 20, 2015 at 16:57
  • $\begingroup$ I'm pretty sure it's the second use-case listed at the top of the Series documentation page. $\endgroup$
    – march
    Nov 20, 2015 at 16:58
  • $\begingroup$ @march But the output it's not in the desired form. $\endgroup$
    – Vaggelis_Z
    Nov 20, 2015 at 16:59

1 Answer 1

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One possibility is:

   se[n_Integer, x0_:1] := 
 Collect[(Normal[
       O[la]^(n + 1) + 
         Expand[Normal[
           Series[V, {x, x0, n}, {y, 0, n}, {z, 0, n}]]] /. {x -> la x,
          y -> la y, z -> la z}] /. la -> 1) /. x -> (x0 + XM), XM] /. 
  XM -> (x - x0)
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  • $\begingroup$ If the point is P(x0,0,0) how would be the general code? $\endgroup$
    – Vaggelis_Z
    Nov 20, 2015 at 17:27

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