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Question: Expanding Derivatives in Equations

Question:Expanding Derivatives in Equations

JohannesC 30 Maple 2021
expand differentialgeometry tensor
November 21 2022
1

Hi Everyone, 

ich have a question, that is probably rather trivial. I am currently doing some calculations with the differential geometry package. (I know the Physics package was recommended, however, the DG package seems more appropriate for my purposes after having a look through the Physics help page)

My goal is to substitute a functional expression into equations derived in curvilinear (spherical and helical) coordinates. my problem now is that maple won't expand the differentials of the expressions I have substituted. 

I hope the answer isn't too trivial. Although I assume it will be :) 

> 

NULL
> 

restart
> 

with(DifferentialGeometry); with(Tensor); with(Tools); Preferences("ShowFramePrompt", false); with(VectorCalculus); BasisFormat(false)
``
> 

DGsetup([r, theta, phi], M)

DGsetup([r, theta, phi], M)

 (1)

Defining the Metric Tensor and Calculating the Connection Coefficients

 
> 

g := evalDG(`&t`(dr, dr)+r^2*sin(phi)^2*`&t`(dtheta, dtheta)+r^2*`&t`(dphi, dphi))

_DG([["tensor", M, [["cov_bas", "cov_bas"], []]], [[[1, 1], 1], [[2, 2], r^2*sin(phi)^2], [[3, 3], r^2]]])

(1.1)
> 

g_inv := InverseMetric(g)

_DG([["tensor", M, [["con_bas", "con_bas"], []]], [[[1, 1], 1], [[2, 2], 1/(r^2*sin(phi)^2)], [[3, 3], 1/r^2]]])

(1.2)
> 

C := Christoffel(g)

> 

DGinfo(C, "ObjectComponents")

[[[1, 2, 2], -r*sin(phi)^2], [[1, 3, 3], -r], [[2, 1, 2], 1/r], [[2, 2, 1], 1/r], [[2, 2, 3], cos(phi)/sin(phi)], [[2, 3, 2], cos(phi)/sin(phi)], [[3, 1, 3], 1/r], [[3, 2, 2], -sin(phi)*cos(phi)], [[3, 3, 1], 1/r]]

 (1.3)

Defining the PDF as a Tensor (Metric) Density

 
> 

f := f__1(r, theta, phi)

f__1(r, theta, phi)

(2.1)
> 

rho := MetricDensity(g, -1)

_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])

(2.2)
> 

Tools:-DGinfo(rho, "TensorDensityType")

Convective Operator

 
> 

CONV := CovariantDerivative(v*f, C)

> 

CONV_1 := simplify(ContractIndices(CONV, [[1, 2]]))

> 

CONV_2 := expand(subs(f__1(r, theta, phi) = f__2(r, theta, phi)*rho, CONV_1))

2*f__2(r, theta, phi)*_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])*v__r/r+v__phi*(diff(f__2(r, theta, phi), phi))*_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])+v__phi*f__2(r, theta, phi)*(diff(_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]]), phi))+cos(phi)*f__2(r, theta, phi)*_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])*v__phi/sin(phi)+(diff(f__2(r, theta, phi), theta))*_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])*v__theta+v__r*(diff(f__2(r, theta, phi), r))*_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]])+v__r*f__2(r, theta, phi)*(diff(_DG([["tensor", M, [[], [["bas", -1]]]], [[[], 1/(r^4*sin(phi)^2)^(1/2)]]]), r))

 (3.1)
> 

NULL

In particular, the last line should simplify quite a bit when expanding derivatives.

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