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`ctensor`

is a component tensor manipulation package. To use the `ctensor`

package, type `load(ctensor)`

.
To begin an interactive session with `ctensor`

, type `csetup()`

. You are
first asked to specify the dimension of the manifold. If the dimension
is 2, 3 or 4 then the list of coordinates defaults to `[x,y]`

, `[x,y,z]`

or `[x,y,z,t]`

respectively.
These names may be changed by assigning a new list of coordinates to
the variable `ct_coords`

(described below) and the user is queried about
this.
** Care must be taken to avoid the coordinate names conflicting
with other object definitions **.

Next, the user enters the metric either directly or from a file by
specifying its ordinal position. As an example of a file of common
metrics, see `share/tensor/metrics.mac`

. The metric is stored in the matrix
LG. Finally, the metric inverse is computed and stored in the matrix
UG. One has the option of carrying out all calculations in a power
series.

A sample protocol is begun below for the static, spherically symmetric
metric (standard coordinates) which will be applied to the problem of
deriving Einstein's vacuum equations (which lead to the Schwarzschild
solution) as an example. Many of the functions in `ctensor`

will be
displayed for the standard metric as examples.

(%i1) load(ctensor); (%o1) /usr/local/lib/maxima/share/tensor/ctensor.mac (%i2) csetup(); Enter the dimension of the coordinate system: 4; Do you wish to change the coordinate names? n; Do you want to 1. Enter a new metric? 2. Enter a metric from a file? 3. Approximate a metric with a Taylor series? 1; Is the matrix 1. Diagonal 2. Symmetric 3. Antisymmetric 4. General Answer 1, 2, 3 or 4 1; Row 1 Column 1: a; Row 2 Column 2: x^2; Row 3 Column 3: x^2*sin(y)^2; Row 4 Column 4: -d; Matrix entered. Enter functional dependencies with the DEPENDS function or 'N' if none depends([a,d],x); Do you wish to see the metric? y; [ a 0 0 0 ] [ ] [ 2 ] [ 0 x 0 0 ] [ ] [ 2 2 ] [ 0 0 x sin (y) 0 ] [ ] [ 0 0 0 - d ] (%o2) done (%i3) christof(mcs); a x (%t3) mcs = --- 1, 1, 1 2 a 1 (%t4) mcs = - 1, 2, 2 x 1 (%t5) mcs = - 1, 3, 3 x d x (%t6) mcs = --- 1, 4, 4 2 d x (%t7) mcs = - - 2, 2, 1 a cos(y) (%t8) mcs = ------ 2, 3, 3 sin(y) 2 x sin (y) (%t9) mcs = - --------- 3, 3, 1 a (%t10) mcs = - cos(y) sin(y) 3, 3, 2 d x (%t11) mcs = --- 4, 4, 1 2 a (%o11) done

__Function:__**csetup***()*-
A function in the
`ctensor`

(component tensor) package which initializes the package and allows the user to enter a metric interactively. See`ctensor`

for more details.

__Function:__**cmetric***(*`dis`)__Function:__**cmetric***()*-
A function in the
`ctensor`

(component tensor) package that computes the metric inverse and sets up the package for further calculations.If

`cframe_flag`

is false, the function computes the inverse metric ug from the (user-defined) matrix`lg`

. The metric determinant is also computed and stored in the variable`gdet`

. Furthermore, the package determines if the metric is diagonal and sets the value of`diagmetric`

accordingly. If the optional argument`dis`is present and not equal to false, the user is prompted to see the metric inverse.If

`cframe_flag`

is true, the function expects that the values of`fri`

(the inverse frame matrix) and`lfg`

(the frame metric) are defined. From these, the frame matrix`fr`

and the inverse frame metric`ufg`

are computed.

__Function:__**ct_coordsys***(*`coordinate_system`,`extra_arg`)__Function:__**ct_coordsys***(*`coordinate_system`)-
Sets up a predefined coordinate system and metric. The argument
`coordinate_system`can be one of the following symbols:SYMBOL Dim Coordinates Description/comments -------------------------------------------------------------------------- cartesian2d 2 [x,y] Cartesian 2D coordinate system polar 2 [r,phi] Polar coordinate system elliptic 2 [u,v] confocalelliptic 2 [u,v] bipolar 2 [u,v] parabolic 2 [u,v] cartesian3d 3 [x,y,z] Cartesian 3D coordinate system polarcylindrical 3 [r,theta,z] ellipticcylindrical 3 [u,v,z] Elliptic 2D with cylindrical Z confocalellipsoidal 3 [u,v,w] bipolarcylindrical 3 [u,v,z] Bipolar 2D with cylintrical Z paraboliccylindrical 3 [u,v,z] Parabolic 2D with cylindrical Z paraboloidal 3 [u,v,phi] conical 3 [u,v,w] toroidal 3 [u,v,phi] spherical 3 [r,theta,phi] Spherical coordinate system oblatespheroidal 3 [u,v,phi] oblatespheroidalsqrt 3 [u,v,phi] prolatespheroidal 3 [u,v,phi] prolatespheroidalsqrt 3 [u,v,phi] ellipsoidal 3 [r,theta,phi] cartesian4d 4 [x,y,z,t] Cartesian 4D coordinate system spherical4d 4 [r,theta,eta,phi] exteriorschwarzschild 4 [t,r,theta,phi] Schwarzschild metric interiorschwarzschild 4 [t,z,u,v] Interior Schwarzschild metric kerr_newman 4 [t,r,theta,phi] Charged axially symmetric metric

`coordinate_system`

can also be a list of transformation functions, followed by a list containing the coordinate variables. For instance, you can specify a spherical metric as follows:(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) ct_coordsys([r*cos(theta)*cos(phi),r*cos(theta)*sin(phi), r*sin(theta),[r,theta,phi]]); (%o2) done (%i3) lg:trigsimp(lg); [ 1 0 0 ] [ ] [ 2 ] (%o3) [ 0 r 0 ] [ ] [ 2 2 ] [ 0 0 r cos (theta) ] (%i4) ct_coords; (%o4) [r, theta, phi] (%i5) dim; (%o5) 3

Transformation functions can also be used when

`cframe_flag`

is`true`

:(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) cframe_flag:true; (%o2) true (%i3) ct_coordsys([r*cos(theta)*cos(phi),r*cos(theta)*sin(phi), r*sin(theta),[r,theta,phi]]); (%o3) done (%i4) fri; [ cos(phi) cos(theta) - cos(phi) r sin(theta) - sin(phi) r cos(theta) ] [ ] (%o4) [ sin(phi) cos(theta) - sin(phi) r sin(theta) cos(phi) r cos(theta) ] [ ] [ sin(theta) r cos(theta) 0 ] (%i5) cmetric(); (%o5) false (%i6) lg:trigsimp(lg); [ 1 0 0 ] [ ] [ 2 ] (%o6) [ 0 r 0 ] [ ] [ 2 2 ] [ 0 0 r cos (theta) ]

The optional argument

`extra_arg`can be any one of the following:`cylindrical`

tells`ct_coordsys`

to attach an additional cylindrical coordinate.`minkowski`

tells`ct_coordsys`

to attach an additional coordinate with negative metric signature.`all`

tells`ct_coordsys`

to call`cmetric`

and`christof(false)`

after setting up the metric.If the global variable

`verbose`

is set to`true`

,`ct_coordsys`

displays the values of`dim`

,`ct_coords`

, and either`lg`

or`lfg`

and`fri`

, depending on the value of`cframe_flag`

.

__Function:__**init_ctensor***()*-
Initializes the
`ctensor`

package.The

`init_ctensor`

function reinitializes the`ctensor`

package. It removes all arrays and matrices used by`ctensor`

, resets all flags, resets`dim`

to 4, and resets the frame metric to the Lorentz-frame.

The main purpose of the `ctensor`

package is to compute the tensors
of curved space(time), most notably the tensors used in general
relativity.

When a metric base is used, `ctensor`

can compute the following tensors:

lg -- ug \ \ lcs -- mcs -- ric -- uric \ \ \ \ tracer - ein -- lein \ riem -- lriem -- weyl \ uriem

`ctensor`

can also work using moving frames. When `cframe_flag`

is
set to `true`

, the following tensors can be calculated:

lfg -- ufg \ fri -- fr -- lcs -- mcs -- lriem -- ric -- uric \ | \ \ \ lg -- ug | weyl tracer - ein -- lein |\ | riem | \uriem

__Function:__**christof***(*`dis`)-
A function in the
`ctensor`

(component tensor) package. It computes the Christoffel symbols of both kinds. The argument`dis`determines which results are to be immediately displayed. The Christoffel symbols of the first and second kinds are stored in the arrays`lcs[i,j,k]`

and`mcs[i,j,k]`

respectively and defined to be symmetric in the first two indices. If the argument to`christof`

is`lcs`

or`mcs`

then the unique non-zero values of`lcs[i,j,k]`

or`mcs[i,j,k]`

, respectively, will be displayed. If the argument is`all`

then the unique non-zero values of`lcs[i,j,k]`

and`mcs[i,j,k]`

will be displayed. If the argument is`false`

then the display of the elements will not occur. The array elements`mcs[i,j,k]`

are defined in such a manner that the final index is contravariant.

__Function:__**ricci***(*`dis`)-
A function in the
`ctensor`

(component tensor) package.`ricci`

computes the covariant (symmetric) components`ric[i,j]`

of the Ricci tensor. If the argument`dis`is`true`

, then the non-zero components are displayed.

__Function:__**uricci***(*`dis`)-
This function first computes the
covariant components
`ric[i,j]`

of the Ricci tensor. Then the mixed Ricci tensor is computed using the contravariant metric tensor. If the value of the argument`dis`is`true`

, then these mixed components,`uric[i,j]`

(the index i is covariant and the index j is contravariant), will be displayed directly. Otherwise,`ricci(false)`

will simply compute the entries of the array`uric[i,j]`

without displaying the results.

__Function:__**scurvature***()*-
returns the scalar curvature (obtained by contracting the Ricci tensor) of the Riemannian manifold with the given metric.

__Function:__**einstein***(*`dis`)-
A function in the
`ctensor`

(component tensor) package.`einstein`

computes the mixed Einstein tensor after the Christoffel symbols and Ricci tensor have been obtained (with the functions`christof`

and`ricci`

). If the argument`dis`is`true`

, then the non-zero values of the mixed Einstein tensor`ein[i,j]`

will be displayed where`j`

is the contravariant index. The variable`rateinstein`

will cause the rational simplification on these components. If`ratfac`

is`true`

then the components will also be factored.

__Function:__**leinstein***(*`dis`)-
Covariant Einstein-tensor.
`leinstein`

stores the values of the covariant Einstein tensor in the array`lein`

. The covariant Einstein-tensor is computed from the mixed Einstein tensor`ein`

by multiplying it with the metric tensor. If the argument`dis`is`true`

, then the non-zero values of the covariant Einstein tensor are displayed.

__Function:__**riemann***(*`dis`)-
A function in the
`ctensor`

(component tensor) package.`riemann`

computes the Riemann curvature tensor from the given metric and the corresponding Christoffel symbols. The following index conventions are used:l _l _l _l _m _l _m R[i,j,k,l] = R = | - | + | | - | | ijk ij,k ik,j mk ij mj ik

This notation is consistent with the notation used by the ITENSOR package and its

`icurvature`

function. If the optional argument`dis`is`true`

, the non-zero components`riem[i,j,k,l]`

will be displayed. As with the Einstein tensor, various switches set by the user control the simplification of the components of the Riemann tensor. If`ratriemann`

is`true`

, then rational simplification will be done. If`ratfac`

is`true`

then each of the components will also be factored.If the variable

`cframe_flag`

is`false`

, the Riemann tensor is computed directly from the Christoffel-symbols. If`cframe_flag`

is`false`

, the covariant Riemann-tensor is computed first from the frame field coefficients.

__Function:__**lriemann***(*`dis`)-
Covariant Riemann-tensor (
`lriem[]`

).Computes the covariant Riemann-tensor as the array

`lriem`

. If the argument`dis`is`true`

, unique nonzero values are displayed.If the variable

`cframe_flag`

is`true`

, the covariant Riemann tensor is computed directly from the frame field coefficients. Otherwise, the (3,1) Riemann tensor is computed first.For information on index ordering, see

`riemann`

.

__Function:__**uriemann***(*`dis`)-
Computes the contravariant components of the Riemann
curvature tensor as array elements
`uriem[i,j,k,l]`

. These are displayed if`dis`is`true`

.

__Function:__**rinvariant***()*-
Forms the Kretchmann-invariant (
`kinvariant`

) obtained by contracting the tensorslriem[i,j,k,l]*uriem[i,j,k,l].

This object is not automatically simplified since it can be very large.

__Function:__**weyl***(*`dis`)-
Computes the Weyl conformal tensor. If the argument
`dis`is`true`

, the non-zero components`weyl[i,j,k,l]`

will be displayed to the user. Otherwise, these components will simply be computed and stored. If the switch`ratweyl`

is set to`true`

, then the components will be rationally simplified; if`ratfac`

is`true`

then the results will be factored as well.

The `ctensor`

package has the ability to truncate results by assuming
that they are Taylor-series approximations. This behavior is controlled by
the `ctayswitch`

variable; when set to true, `ctensor`

makes use
internally of the function `ctaylor`

when simplifying results.

The `ctaylor`

function is invoked by the following `ctensor`

functions:

Function Comments --------------------------------- christof() For mcs only ricci() uricci() einstein() riemann() weyl() checkdiv()

__Function:__**ctaylor***()*-
The

`ctaylor`

function truncates its argument by converting it to a Taylor-series using`taylor`

, and then calling`ratdisrep`

. This has the combined effect of dropping terms higher order in the expansion variable`ctayvar`

. The order of terms that should be dropped is defined by`ctaypov`

; the point around which the series expansion is carried out is specified in`ctaypt`

.As an example, consider a simple metric that is a perturbation of the Minkowski metric. Without further restrictions, even a diagonal metric produces expressions for the Einstein tensor that are far too complex:

(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) ratfac:true; (%o2) true (%i3) derivabbrev:true; (%o3) true (%i4) ct_coords:[t,r,theta,phi]; (%o4) [t, r, theta, phi] (%i5) lg:matrix([-1,0,0,0],[0,1,0,0],[0,0,r^2,0],[0,0,0,r^2*sin(theta)^2]); [ - 1 0 0 0 ] [ ] [ 0 1 0 0 ] [ ] (%o5) [ 2 ] [ 0 0 r 0 ] [ ] [ 2 2 ] [ 0 0 0 r sin (theta) ] (%i6) h:matrix([h11,0,0,0],[0,h22,0,0],[0,0,h33,0],[0,0,0,h44]); [ h11 0 0 0 ] [ ] [ 0 h22 0 0 ] (%o6) [ ] [ 0 0 h33 0 ] [ ] [ 0 0 0 h44 ] (%i7) depends(l,r); (%o7) [l(r)] (%i8) lg:lg+l*h; [ h11 l - 1 0 0 0 ] [ ] [ 0 h22 l + 1 0 0 ] [ ] (%o8) [ 2 ] [ 0 0 r + h33 l 0 ] [ ] [ 2 2 ] [ 0 0 0 r sin (theta) + h44 l ] (%i9) cmetric(false); (%o9) done (%i10) einstein(false); (%o10) done (%i11) ntermst(ein); [[1, 1], 62] [[1, 2], 0] [[1, 3], 0] [[1, 4], 0] [[2, 1], 0] [[2, 2], 24] [[2, 3], 0] [[2, 4], 0] [[3, 1], 0] [[3, 2], 0] [[3, 3], 46] [[3, 4], 0] [[4, 1], 0] [[4, 2], 0] [[4, 3], 0] [[4, 4], 46] (%o12) done

However, if we recompute this example as an approximation that is linear in the variable

`l`

, we get much simpler expressions:(%i14) ctayswitch:true; (%o14) true (%i15) ctayvar:l; (%o15) l (%i16) ctaypov:1; (%o16) 1 (%i17) ctaypt:0; (%o17) 0 (%i18) christof(false); (%o18) done (%i19) ricci(false); (%o19) done (%i20) einstein(false); (%o20) done (%i21) ntermst(ein); [[1, 1], 6] [[1, 2], 0] [[1, 3], 0] [[1, 4], 0] [[2, 1], 0] [[2, 2], 13] [[2, 3], 2] [[2, 4], 0] [[3, 1], 0] [[3, 2], 2] [[3, 3], 9] [[3, 4], 0] [[4, 1], 0] [[4, 2], 0] [[4, 3], 0] [[4, 4], 9] (%o21) done (%i22) ratsimp(ein[1,1]); 2 2 4 2 2 (%o22) - (((h11 h22 - h11 ) (l ) r - 2 h33 l r ) sin (theta) r r r 2 2 4 2 - 2 h44 l r - h33 h44 (l ) )/(4 r sin (theta)) r r r

This capability can be useful, for instance, when working in the weak field limit far from a gravitational source.

When the variable `cframe_flag`

is set to true, the `ctensor`

package
performs its calculations using a moving frame.

__Function:__**frame_bracket***(*`fr`,`fri`,`diagframe`)-
The frame bracket (
`fb[]`

).Computes the frame bracket according to the following definition:

c c c d e ifb = ( ifri - ifri ) ifr ifr ab d,e e,d a b

A new feature (as of November, 2004) of `ctensor`

is its ability to
compute the Petrov classification of a 4-dimensional spacetime metric.
For a demonstration of this capability, see the file
`share/tensor/petrov.dem`

.

__Function:__**nptetrad***()*-
Computes a Newman-Penrose null tetrad (
`np`

) and its raised-index counterpart (`npi`

). See`petrov`

for an example.The null tetrad is constructed on the assumption that a four-diemensional orthonormal frame metric with metric signature (-,+,+,+) is being used. The components of the null tetrad are related to the inverse frame matrix as follows:

np = (fri + fri ) / sqrt(2) 1 1 2 np = (fri - fri ) / sqrt(2) 2 1 2 np = (fri + %i fri ) / sqrt(2) 3 3 4 np = (fri - %i fri ) / sqrt(2) 4 3 4

__Function:__**psi***(*`dis`)-
Computes the five Newman-Penrose coefficients
`psi[0]`

...`psi[4]`

. If`psi`

is set to`true`

, the coefficients are displayed. See`petrov`

for an example.These coefficients are computed from the Weyl-tensor in a coordinate base. If a frame base is used, the Weyl-tensor is first converted to a coordinate base, which can be a computationally expensive procedure. For this reason, in some cases it may be more advantageous to use a coordinate base in the first place before the Weyl tensor is computed. Note however, that constructing a Newman-Penrose null tetrad requires a frame base. Therefore, a meaningful computation sequence may begin with a frame base, which is then used to compute

`lg`

(computed automatically by`cmetric`

and then`ug`

. At this point, you can switch back to a coordinate base by setting`cframe_flag`

to false before beginning to compute the Christoffel symbols. Changing to a frame base at a later stage could yield inconsistent results, as you may end up with a mixed bag of tensors, some computed in a frame base, some in a coordinate base, with no means to distinguish between the two.

__Function:__**petrov***()*-
Computes the Petrov classification of the metric characterized by
`psi[0]`

...`psi[4]`

.For example, the following demonstrates how to obtain the Petrov-classification of the Kerr metric:

(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) (cframe_flag:true,gcd:spmod,ctrgsimp:true,ratfac:true); (%o2) true (%i3) ct_coordsys(exteriorschwarzschild,all); (%o3) done (%i4) ug:invert(lg)$ (%i5) weyl(false); (%o5) done (%i6) nptetrad(true); (%t6) np = [ sqrt(r - 2 m) sqrt(r) ] [ --------------- --------------------- 0 0 ] [ sqrt(2) sqrt(r) sqrt(2) sqrt(r - 2 m) ] [ ] [ sqrt(r - 2 m) sqrt(r) ] [ --------------- - --------------------- 0 0 ] [ sqrt(2) sqrt(r) sqrt(2) sqrt(r - 2 m) ] [ ] [ r %i r sin(theta) ] [ 0 0 ------- --------------- ] [ sqrt(2) sqrt(2) ] [ ] [ r %i r sin(theta) ] [ 0 0 ------- - --------------- ] [ sqrt(2) sqrt(2) ] sqrt(r) sqrt(r - 2 m) (%t7) npi = matrix([- ---------------------, ---------------, 0, 0], sqrt(2) sqrt(r - 2 m) sqrt(2) sqrt(r) sqrt(r) sqrt(r - 2 m) [- ---------------------, - ---------------, 0, 0], sqrt(2) sqrt(r - 2 m) sqrt(2) sqrt(r) 1 %i [0, 0, ---------, --------------------], sqrt(2) r sqrt(2) r sin(theta) 1 %i [0, 0, ---------, - --------------------]) sqrt(2) r sqrt(2) r sin(theta) (%o7) done (%i7) psi(true); (%t8) psi = 0 0 (%t9) psi = 0 1 m (%t10) psi = -- 2 3 r (%t11) psi = 0 3 (%t12) psi = 0 4 (%o12) done (%i12) petrov(); (%o12) D

The Petrov classification function is based on the algorithm published in "Classifying geometries in general relativity: III Classification in practice" by Pollney, Skea, and d'Inverno, Class. Quant. Grav. 17 2885-2902 (2000). Except for some simple test cases, the implementation is untested as of December 19, 2004, and is likely to contain errors.

`ctensor`

has the ability to compute and include torsion and nonmetricity
coefficients in the connection coefficients.

The torsion coefficients are calculated from a user-supplied tensor
`tr`

, which should be a rank (2,1) tensor. From this, the torsion
coefficients `kt`

are computed according to the following formulae:

m m m - g tr - g tr - tr g im kj jm ki ij km kt = ------------------------------- ijk 2 k km kt = g kt ij ijm

Note that only the mixed-index tensor is calculated and stored in the
array `kt`

.

The nonmetricity coefficients are calculated from the user-supplied
nonmetricity vector `nm`

. From this, the nonmetricity coefficients
`nmc`

are computed as follows:

k k km -nm D - D nm + g nm g k i j i j m ij nmc = ------------------------------ ij 2

where D stands for the Kronecker-delta.

When `ctorsion_flag`

is set to `true`

, the values of `kt`

are substracted from the mixed-indexed connection coefficients computed by
`christof`

and stored in `mcs`

. Similarly, if `cnonmet_flag`

is set to `true`

, the values of `nmc`

are substracted from the
mixed-indexed connection coefficients.

If necessary, `christof`

calls the functions `contortion`

and
`nonmetricity`

in order to compute `kt`

and `nm`

.

__Function:__**contortion***(*`tr`)-
Computes the (2,1) contortion coefficients from the torsion tensor

`tr`.

__Function:__**nonmetricity***(*`nm`)-
Computes the (2,1) nonmetricity coefficients from the nonmetricity vector

`nm`.

__Function:__**ctransform***(*`M`)-
A function in the
`ctensor`

(component tensor) package which will perform a coordinate transformation upon an arbitrary square symmetric matrix`M`. The user must input the functions which define the transformation. (Formerly called`transform`

.)

__Function:__**findde***(*`A`,`n`)-
returns a list of the unique differential equations (expressions) corresponding to the elements of the

`n`dimensional square array`A`. Presently,`n`may be 2 or 3.`deindex`

is a global list containing the indices of`A`corresponding to these unique differential equations. For the Einstein tensor (`ein`

), which is a two dimensional array, if computed for the metric in the example below,`findde`

gives the following independent differential equations:(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) derivabbrev:true; (%o2) true (%i3) dim:4; (%o3) 4 (%i4) lg:matrix([a,0,0,0],[0,x^2,0,0],[0,0,x^2*sin(y)^2,0],[0,0,0,-d]); [ a 0 0 0 ] [ ] [ 2 ] [ 0 x 0 0 ] (%o4) [ ] [ 2 2 ] [ 0 0 x sin (y) 0 ] [ ] [ 0 0 0 - d ] (%i5) depends([a,d],x); (%o5) [a(x), d(x)] (%i6) ct_coords:[x,y,z,t]; (%o6) [x, y, z, t] (%i7) cmetric(); (%o7) done (%i8) einstein(false); (%o8) done (%i9) findde(ein,2); 2 (%o9) [d x - a d + d, 2 a d d x - a (d ) x - a d d x + 2 a d d x x x x x x x 2 2 - 2 a d , a x + a - a] x x (%i10) deindex; (%o10) [[1, 1], [2, 2], [4, 4]]

__Function:__**cograd***()*-
Computes the covariant gradient of a scalar function allowing the
user to choose the corresponding vector name as the example under
`contragrad`

illustrates.

__Function:__**contragrad***()*-
Computes the contravariant gradient of a scalar function allowing the user to choose the corresponding vector name as the example below for the Schwarzschild metric illustrates:

(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) derivabbrev:true; (%o2) true (%i3) ct_coordsys(exteriorschwarzschild,all); (%o3) done (%i4) depends(f,r); (%o4) [f(r)] (%i5) cograd(f,g1); (%o5) done (%i6) listarray(g1); (%o6) [0, f , 0, 0] r (%i7) contragrad(f,g2); (%o7) done (%i8) listarray(g2); f r - 2 f m r r (%o8) [0, -------------, 0, 0] r

__Function:__**dscalar***()*-
computes the tensor d'Alembertian of the scalar function once
dependencies have been declared upon the function. For example:
(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) derivabbrev:true; (%o2) true (%i3) ct_coordsys(exteriorschwarzschild,all); (%o3) done (%i4) depends(p,r); (%o4) [p(r)] (%i5) factor(dscalar(p)); 2 p r - 2 m p r + 2 p r - 2 m p r r r r r r (%o5) -------------------------------------- 2 r

__Function:__**checkdiv***()*-
computes the covariant divergence of the mixed second rank tensor (whose first index must be covariant) by printing the corresponding n components of the vector field (the divergence) where n =

`dim`

. If the argument to the function is`g`

then the divergence of the Einstein tensor will be formed and must be zero. In addition, the divergence (vector) is given the array name`div`

.

__Function:__**cgeodesic***(*`dis`)-
A function in the
`ctensor`

(component tensor) package.`cgeodesic`

computes the geodesic equations of motion for a given metric. They are stored in the array`geod[i]`

. If the argument`dis`is`true`

then these equations are displayed.

__Function:__**bdvac***(*`f`)-
generates the covariant components of the vacuum field equations of the Brans- Dicke gravitational theory. The scalar field is specified by the argument

`f`, which should be a (quoted) function name with functional dependencies, e.g.,`'p(x)`

.The components of the second rank covariant field tensor are represented by the array

`bd`

.

__Function:__**invariant1***()*-
generates the mixed Euler- Lagrange tensor (field equations) for the invariant density of R^2. The field equations are the components of an array named

`inv1`

.

__Function:__**invariant2***()*-
*** NOT YET IMPLEMENTED ***

generates the mixed Euler- Lagrange tensor (field equations) for the invariant density of

`ric[i,j]*uriem[i,j]`

. The field equations are the components of an array named`inv2`

.

__Function:__**bimetric***()*-
*** NOT YET IMPLEMENTED ***

generates the field equations of Rosen's bimetric theory. The field equations are the components of an array named

`rosen`

.

__Function:__**diagmatrixp***(*`M`)-
Returns

`true`

if`M`is a diagonal matrix or (2D) array.

__Function:__**symmetricp***(*`M`)-
Returns

`true`

if`M`is a symmetric matrix or (2D) array.

__Function:__**ntermst***(*`f`)-
gives the user a quick picture of the "size" of the doubly subscripted
tensor (array)
`f`. It prints two element lists where the second element corresponds to NTERMS of the components specified by the first elements. In this way, it is possible to quickly find the non-zero expressions and attempt simplification.

__Function:__**cdisplay***(*`ten`)-
displays all the elements of the tensor
`ten`, as represented by a multidimensional array. Tensors of rank 0 and 1, as well as other types of variables, are displayed as with`ldisplay`

. Tensors of rank 2 are displayed as 2-dimensional matrices, while tensors of higher rank are displayed as a list of 2-dimensional matrices. For instance, the Riemann-tensor of the Schwarzschild metric can be viewed as:(%i1) load(ctensor); (%o1) /share/tensor/ctensor.mac (%i2) ratfac:true; (%o2) true (%i3) ct_coordsys(exteriorschwarzschild,all); (%o3) done (%i4) riemann(false); (%o4) done (%i5) cdisplay(riem); [ 0 0 0 0 ] [ ] [ 2 ] [ 3 m (r - 2 m) m 2 m ] [ 0 - ------------- + -- - ---- 0 0 ] [ 4 3 4 ] [ r r r ] [ ] riem = [ m (r - 2 m) ] 1, 1 [ 0 0 ----------- 0 ] [ 4 ] [ r ] [ ] [ m (r - 2 m) ] [ 0 0 0 ----------- ] [ 4 ] [ r ] [ 2 m (r - 2 m) ] [ 0 ------------- 0 0 ] [ 4 ] [ r ] riem = [ ] 1, 2 [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ m (r - 2 m) ] [ 0 0 - ----------- 0 ] [ 4 ] [ r ] riem = [ ] 1, 3 [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ m (r - 2 m) ] [ 0 0 0 - ----------- ] [ 4 ] [ r ] riem = [ ] 1, 4 [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ 0 0 0 0 ] [ ] [ 2 m ] [ - ------------ 0 0 0 ] riem = [ 2 ] 2, 1 [ r (r - 2 m) ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ 2 m ] [ ------------ 0 0 0 ] [ 2 ] [ r (r - 2 m) ] [ ] [ 0 0 0 0 ] [ ] riem = [ m ] 2, 2 [ 0 0 - ------------ 0 ] [ 2 ] [ r (r - 2 m) ] [ ] [ m ] [ 0 0 0 - ------------ ] [ 2 ] [ r (r - 2 m) ] [ 0 0 0 0 ] [ ] [ m ] [ 0 0 ------------ 0 ] riem = [ 2 ] 2, 3 [ r (r - 2 m) ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ 0 0 0 0 ] [ ] [ m ] [ 0 0 0 ------------ ] riem = [ 2 ] 2, 4 [ r (r - 2 m) ] [ ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ m ] 3, 1 [ - 0 0 0 ] [ r ] [ ] [ 0 0 0 0 ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ m ] 3, 2 [ 0 - 0 0 ] [ r ] [ ] [ 0 0 0 0 ] [ m ] [ - - 0 0 0 ] [ r ] [ ] [ m ] [ 0 - - 0 0 ] riem = [ r ] 3, 3 [ ] [ 0 0 0 0 ] [ ] [ 2 m - r ] [ 0 0 0 ------- + 1 ] [ r ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ 2 m ] 3, 4 [ 0 0 0 - -- ] [ r ] [ ] [ 0 0 0 0 ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ 0 0 0 0 ] 4, 1 [ ] [ 2 ] [ m sin (theta) ] [ ------------- 0 0 0 ] [ r ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ 0 0 0 0 ] 4, 2 [ ] [ 2 ] [ m sin (theta) ] [ 0 ------------- 0 0 ] [ r ] [ 0 0 0 0 ] [ ] [ 0 0 0 0 ] [ ] riem = [ 0 0 0 0 ] 4, 3 [ ] [ 2 ] [ 2 m sin (theta) ] [ 0 0 - --------------- 0 ] [ r ] [ 2 ] [ m sin (theta) ] [ - ------------- 0 0 0 ] [ r ] [ ] [ 2 ] [ m sin (theta) ] riem = [ 0 - ------------- 0 0 ] 4, 4 [ r ] [ ] [ 2 ] [ 2 m sin (theta) ] [ 0 0 --------------- 0 ] [ r ] [ ] [ 0 0 0 0 ] (%o5) done

__Function:__**deleten***(*`L`,`n`)-
Returns a new list consisting of
`L`with the`n`'th element deleted.

`ctensor`

__Option variable:__**dim**-
Default value: 4
An option in the

`ctensor`

(component tensor) package.`dim`

is the dimension of the manifold with the default 4. The command`dim: n`

will reset the dimension to any other value`n`

.

__Option variable:__**diagmetric**-
Default value:
`false`

An option in the

`ctensor`

(component tensor) package. If`diagmetric`

is`true`

special routines compute all geometrical objects (which contain the metric tensor explicitly) by taking into consideration the diagonality of the metric. Reduced run times will, of course, result. Note: this option is set automatically by`csetup`

if a diagonal metric is specified.

__Option variable:__**ctrgsimp**-
Causes trigonometric simplifications to be used when tensors are computed. Presently,

`ctrgsimp`

affects only computations involving a moving frame.

__Option variable:__**cframe_flag**-
Causes computations to be performed relative to a moving frame as opposed to a holonomic metric. The frame is defined by the inverse frame array

`fri`

and the frame metric`lfg`

. For computations using a Cartesian frame,`lfg`

should be the unit matrix of the appropriate dimension; for computations in a Lorentz frame,`lfg`

should have the appropriate signature.

__Option variable:__**ctorsion_flag**-
Causes the contortion tensor to be included in the computation of the connection coefficients. The contortion tensor itself is computed by

`contortion`

from the user-supplied tensor`tr`

.

__Option variable:__**cnonmet_flag**-
Causes the nonmetricity coefficients to be included in the computation of the connection coefficients. The nonmetricity coefficients are computed from the user-supplied nonmetricity vector

`nm`

by the function`nonmetricity`

.

__Option variable:__**ctayswitch**-
If set to

`true`

, causes some`ctensor`

computations to be carried out using Taylor-series expansions. Presently,`christof`

,`ricci`

,`uricci`

,`einstein`

, and`weyl`

take into account this setting.

__Option variable:__**ctayvar**-
Variable used for Taylor-series expansion if

`ctayswitch`

is set to`true`

.

__Option variable:__**ctaypov**-
Maximum power used in Taylor-series expansion when

`ctayswitch`

is set to`true`

.

__Option variable:__**ctaypt**-
Point around which Taylor-series expansion is carried out when

`ctayswitch`

is set to`true`

.

__System variable:__**gdet**-
The determinant of the metric tensor

`lg`

. Computed by`cmetric`

when`cframe_flag`

is set to`false`

.

__Option variable:__**ratchristof**-
Causes rational simplification to be applied by

`christof`

.

__Option variable:__**rateinstein**-
Default value:
`true`

If

`true`

rational simplification will be performed on the non-zero components of Einstein tensors; if`ratfac`

is`true`

then the components will also be factored.

__Option variable:__**ratriemann**-
Default value:
`true`

One of the switches which controls simplification of Riemann tensors; if

`true`

, then rational simplification will be done; if`ratfac`

is`true`

then each of the components will also be factored.

__Option variable:__**ratweyl**-
Default value:
`true`

If

`true`

, this switch causes the`weyl`

function to apply rational simplification to the values of the Weyl tensor. If`ratfac`

is`true`

, then the components will also be factored.

__Variable:__**lfg**-
The covariant frame metric. By default, it is initialized to the 4-dimensional Lorentz frame with signature (+,+,+,-). Used when
`cframe_flag`

is`true`

.

__Variable:__**ufg**-
The inverse frame metric. Computed from
`lfg`

when`cmetric`

is called while`cframe_flag`

is set to`true`

.

__Variable:__**riem**-
The (3,1) Riemann tensor. Computed when the function
`riemann`

is invoked. For information about index ordering, see the description of`riemann`

.if

`cframe_flag`

is`true`

,`riem`

is computed from the covariant Riemann-tensor`lriem`

.

__Variable:__**lriem**-
The covariant Riemann tensor. Computed by

`lriemann`

.

__Variable:__**uriem**-
The contravariant Riemann tensor. Computed by

`uriemann`

.

__Variable:__**ric**-
The mixed Ricci-tensor. Computed by

`ricci`

.

__Variable:__**uric**-
The contravariant Ricci-tensor. Computed by

`uricci`

.

__Variable:__**lg**-
The metric tensor. This tensor must be specified (as a

`dim`

by`dim`

matrix) before other computations can be performed.

__Variable:__**ug**-
The inverse of the metric tensor. Computed by

`cmetric`

.

__Variable:__**weyl**-
The Weyl tensor. Computed by

`weyl`

.

__Variable:__**fb**-
Frame bracket coefficients, as computed by

`frame_bracket`

.

__Variable:__**kinvariant**-
The Kretchmann invariant. Computed by

`rinvariant`

.

__Variable:__**np**-
A Newman-Penrose null tetrad. Computed by

`nptetrad`

.

__Variable:__**npi**-
The raised-index Newman-Penrose null tetrad. Computed by

`nptetrad`

. Defined as`ug.np`

. The product`np.transpose(npi)`

is constant:(%i39) trigsimp(np.transpose(npi)); [ 0 - 1 0 0 ] [ ] [ - 1 0 0 0 ] (%o39) [ ] [ 0 0 0 1 ] [ ] [ 0 0 1 0 ]

__Variable:__**tr**-
User-supplied rank-3 tensor representing torsion. Used by

`contortion`

.

__Variable:__**kt**-
The contortion tensor, computed from

`tr`

by`contortion`

.

__Variable:__**nm**-
User-supplied nonmetricity vector. Used by

`nonmetricity`

.

__Variable:__**nmc**-
The nonmetricity coefficients, computed from

`nm`

by`nonmetricity`

.

__System variable:__**tensorkill**-
Variable indicating if the tensor package has been initialized. Set and used by

`csetup`

, reset by`init_ctensor`

.

__Option variable:__**ct_coords**-
Default value:
`[]`

An option in the

`ctensor`

(component tensor) package.`ct_coords`

contains a list of coordinates. While normally defined when the function`csetup`

is called, one may redefine the coordinates with the assignment`ct_coords: [j1, j2, ..., jn]`

where the j's are the new coordinate names. See also`csetup`

.

The following names are used internally by the `ctensor`

package and
should not be redefined:

Name Description --------------------------------------- _lg() Evaluates to lfg if frame metric used, lg otherwise _ug() Evaluates to ufg if frame metric used, ug otherwise cleanup() Removes items drom the deindex list contract4() Used by psi() filemet() Used by csetup() when reading the metric from a file findde1() Used by findde() findde2() Used by findde() findde3() Used by findde() kdelt() Kronecker-delta (not generalized) newmet() Used by csetup() for setting up a metric interactively setflags() Used by init_ctensor() readvalue() resimp() sermet() Used by csetup() for entering a metric as Taylor-series txyzsum() tmetric() Frame metric, used by cmetric() when cframe_flag:true triemann() Riemann-tensor in frame base, used when cframe_flag:true tricci() Ricci-tensor in frame base, used when cframe_flag:true trrc() Ricci rotation coefficients, used by christof() yesp()

In November, 2004, the `ctensor`

package was extensively rewritten.
Many functions and variables have been renamed in order to make the
package compatible with the commercial version of Macsyma.

New Name Old Name Description -------------------------------------------------------------------------- ctaylor() DLGTAYLOR() Taylor-series expansion of an expression lgeod[] EM Geodesic equations ein[] G[] Mixed Einstein-tensor ric[] LR[] Mixed Ricci-tensor ricci() LRICCICOM() Compute the mixed Ricci-tensor ctaypov MINP Maximum power in Taylor-series expansion cgeodesic() MOTION Compute geodesic equations ct_coords OMEGA Metric coordinates ctayvar PARAM Taylor-series expansion variable lriem[] R[] Covariant Riemann-tensor uriemann() RAISERIEMANN() Compute the contravariant Riemann-tensor ratriemann RATRIEMAN Rational simplification of the Riemann-tensor uric[] RICCI[] Contravariant Ricci-tensor uricci() RICCICOM() Compute the contravariant Ricci-tensor cmetric() SETMETRIC() Set up the metric ctaypt TAYPT Point for Taylor-series expansion ctayswitch TAYSWITCH Taylor-series setting switch csetup() TSETUP() Start interactive setup session ctransform() TTRANSFORM() Interactive coordinate transformation uriem[] UR[] Contravariant Riemann-tensor weyl[] W[] (3,1) Weyl-tensor

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