Tensor[CovariantlyConstantTensors] - calculate the covariantly constant tensors with respect to a given metric or connection
Calling Sequences
CovariantlyConstantTensors(g, T, options)
CovariantlyConstantTensors(Gamma, T, options )
Parameters
g - a metric tensor on a manifold
Gamma - a connection, either an affine connection on or a connection on a vector bundle over
T - a list of vector fields, differential forms or tensors (all of the same type)
options - any of the following keywords arguments: ansatz, auxiliaryequations, coefficientvariables, unknowns, parameters, output
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Description
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This command is part of the DifferentialGeometry:-Tensor package, and so can be used in the form CovariantlyConstantTensors(...) only after executing the commands with(DifferentialGeometry), with(Tensor) in that order. It can always be used in the long form DifferentialGeometry:-Tensor:-CovariantlyConstantTensors.
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Examples
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Example 1.
We find the covariantly constant 2 forms and covariantly constant rank 2 symmetric tensors for a metric , defined on a 3 dimensional manifold.
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We use the command GenerateForms to generate a basis for the space of 2 forms.
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The space of covariantly constant 2 forms is 1-dimensional.
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We use the command GenerateSymmetricTensors to generate a basis for the space of rank 2 symmetric tensors.
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The space of covariantly constant, rank 2 symmetric tensors is two-dimensional. We obtain the output as a single tensor depending upon two arbitrary constants and
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We can check this result using the CovariantDerivative command. For this we need the Christoffel connection for the metric.
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| (2.7) |
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Example 2.
We find the the trace-free, covariantly constant, rank 2 symmetric tensors for the metric from Example 1. First construct the general rank 2 symmetric tensor.
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The trace of is given by
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We now invoke the keyword arguments ansatz, auxiliaryequations, and unknowns.
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Example 3.
In this example we consider a metric which depends upon an arbitrary function . We find that generically there are no covariantly constant vector fields, but when the function is constant there are 2.
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We use the keyword argument parameters to invoke case-splitting with respect to the function .
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Example 4.
We define a connection on a rank 2 vector bundle over a 3 dimensional base manifold.
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| (2.17) |
We calculate the covariantly constant type tensors on . The command GenerateTensors is used to generate a basis for the tensors.
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The most general tensor on is given by a linear combination of the elements of the list , using coefficients which are functions of the base variables alone. We specify this dependency with the keyword argument coefficientvariables .
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| (2.19) |
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