@menu
* Introduction to atensor::
* Functions and Variables for atensor::
@end menu

@node Introduction to atensor, Functions and Variables for atensor, Package atensor, Package atensor
@section Introduction to atensor

@code{atensor} is an algebraic tensor manipulation package. To use @code{atensor},
type @code{load("atensor")}, followed by a call to the @code{init_atensor}
function.

For alternative packages for handling tensors @pxref{Package ctensor} and @ref{Package itensor}.

The essence of @code{atensor} is a set of simplification rules for the
noncommutative (dot) product operator ("@code{.}"). @code{atensor} recognizes
several algebra types; the corresponding simplification rules are put
into effect when the @code{init_atensor} function is called.

The capabilities of @code{atensor} can be demonstrated by defining the
algebra of quaternions as a Clifford-algebra Cl(0,2) with two basis
vectors. The three quaternionic imaginary units are then the two
basis vectors and their product, i.e.:

@example
    i = v     j = v     k = v  . v
         1         2         1    2
@end example

Although the @code{atensor} package has a built-in definition for the
quaternion algebra, it is not used in this example, in which we
endeavour to build the quaternion multiplication table as a matrix:

@c ===beg===
@c load("atensor");
@c init_atensor(clifford,0,0,2);
@c atensimp(v[1].v[1]);
@c atensimp((v[1].v[2]).(v[1].v[2]));
@c q:zeromatrix(4,4);
@c q[1,1]:1;
@c for i thru adim do q[1,i+1]:q[i+1,1]:v[i];
@c q[1,4]:q[4,1]:v[1].v[2];
@c for i from 2 thru 4 do for j from 2 thru 4 do
@c q[i,j]:atensimp(q[i,1].q[1,j]);
@c q;
@c ===end===
@example
(%i1) load("atensor");
(%o1)       /share/tensor/atensor.mac
(%i2) init_atensor(clifford,0,0,2);
(%o2)                                done
(%i3) atensimp(v[1].v[1]);
(%o3)                                 - 1
(%i4) atensimp((v[1].v[2]).(v[1].v[2]));
(%o4)                                 - 1
(%i5) q:zeromatrix(4,4);
                                [ 0  0  0  0 ]
                                [            ]
                                [ 0  0  0  0 ]
(%o5)                           [            ]
                                [ 0  0  0  0 ]
                                [            ]
                                [ 0  0  0  0 ]
(%i6) q[1,1]:1;
(%o6)                                  1
(%i7) for i thru adim do q[1,i+1]:q[i+1,1]:v[i];
(%o7)                                done
(%i8) q[1,4]:q[4,1]:v[1].v[2];
(%o8)                               v  . v
                                     1    2
(%i9) for i from 2 thru 4 do for j from 2 thru 4 do
      q[i,j]:atensimp(q[i,1].q[1,j]);
(%o9)                                done
(%i10) q;
@group
                   [    1        v         v      v  . v  ]
                   [              1         2      1    2 ]
                   [                                      ]
                   [   v         - 1     v  . v    - v    ]
                   [    1                 1    2      2   ]
(%o10)             [                                      ]
                   [   v      - v  . v     - 1      v     ]
                   [    2        1    2              1    ]
                   [                                      ]
                   [ v  . v      v        - v       - 1   ]
                   [  1    2      2          1            ]
@end group
@end example

@code{atensor} recognizes as base vectors indexed symbols, where the symbol
is that stored in @code{asymbol} and the index runs between 1 and @code{adim}.
For indexed symbols, and indexed symbols only, the bilinear forms
@code{sf}, @code{af}, and @code{av} are evaluated. The evaluation
substitutes the value of @code{aform[i,j]} in place of @code{fun(v[i],v[j])}
where @code{v} represents the value of @code{asymbol} and @code{fun} is
either @code{af} or @code{sf}; or, it substitutes @code{v[aform[i,j]]}
in place of @code{av(v[i],v[j])}.

Needless to say, the functions @code{sf}, @code{af} and @code{av}
can be redefined.

When the @code{atensor} package is loaded, the following flags are set:

@example
dotscrules:true;
dotdistrib:true;
dotexptsimp:false;
@end example

If you wish to experiment with a nonassociative algebra, you may also
consider setting @code{dotassoc} to @code{false}. In this case, however,
@code{atensimp} will not always be able to obtain the desired
simplifications.

@opencatbox{Categories:}
@category{Tensors}
@category{Share packages}
@category{Package atensor}
@closecatbox

@c end concepts atensor
@node Functions and Variables for atensor,  , Introduction to atensor, Package atensor

@section Functions and Variables for atensor

@anchor{init_atensor}
@deffn {Function} init_atensor @
@fname{init_atensor} (@var{alg_type}, @var{opt_dims}) @
@fname{init_atensor} (@var{alg_type})

Initializes the @code{atensor} package with the specified algebra type. @var{alg_type}
can be one of the following:

@code{universal}: The universal algebra has no commutation rules.

@code{grassmann}: The Grassman algebra is defined by the commutation
relation @code{u.v+v.u=0}.

@code{clifford}: The Clifford algebra is defined by the commutation
relation @code{u.v+v.u=-2*sf(u,v)} where @code{sf} is a symmetric
scalar-valued function. For this algebra, @var{opt_dims} can be up
to three nonnegative integers, representing the number of positive,
degenerate, and negative dimensions of the algebra, respectively. If
any @var{opt_dims} values are supplied, @code{atensor} will configure the
values of @code{adim} and @code{aform} appropriately. Otherwise,
@code{adim} will default to 0 and @code{aform} will not be defined.

@code{symmetric}: The symmetric algebra is defined by the commutation
relation @code{u.v-v.u=0}.

@code{symplectic}: The symplectic algebra is defined by the commutation
relation @code{u.v-v.u=2*af(u,v)} where @code{af} is an antisymmetric
scalar-valued function. For the symplectic algebra, @var{opt_dims} can
be up to two nonnegative integers, representing the nondegenerate and
degenerate dimensions, respectively. If any @var{opt_dims} values are
supplied, @code{atensor} will configure the values of @code{adim} and @code{aform}
appropriately. Otherwise, @code{adim} will default to 0 and @code{aform}
will not be defined.

@code{lie_envelop}: The algebra of the Lie envelope is defined by the
commutation relation @code{u.v-v.u=2*av(u,v)} where @code{av} is
an antisymmetric function.

The @code{init_atensor} function also recognizes several predefined
algebra types:

@code{complex} implements the algebra of complex numbers as the
Clifford algebra Cl(0,1). The call @code{init_atensor(complex)} is
equivalent to @code{init_atensor(clifford,0,0,1)}.

@code{quaternion} implements the algebra of quaternions. The call
@code{init_atensor (quaternion)} is equivalent to
@code{init_atensor (clifford,0,0,2)}.

@code{pauli} implements the algebra of Pauli-spinors as the Clifford-algebra
Cl(3,0). A call to @code{init_atensor(pauli)} is equivalent to
@code{init_atensor(clifford,3)}.

@code{dirac} implements the algebra of Dirac-spinors as the Clifford-algebra
Cl(3,1). A call to @code{init_atensor(dirac)} is equivalent to
@code{init_atensor(clifford,3,0,1)}.

@opencatbox{Categories:}
@category{Package atensor}
@closecatbox
@end deffn


@anchor{atensimp}
@deffn {Function} atensimp (@var{expr})

Simplifies an algebraic tensor expression @var{expr} according to the rules
configured by a call to @code{init_atensor}. Simplification includes
recursive application of commutation relations and resolving calls
to @code{sf}, @code{af}, and @code{av} where applicable. A
safeguard is used to ensure that the function always terminates, even
for complex expressions.

@opencatbox{Categories:}
@category{Package atensor}
@category{Simplification functions}
@closecatbox

@end deffn

@anchor{alg_type}
@deffn {Function} alg_type
The algebra type. Valid values are @code{universal}, @code{grassmann},
@code{clifford}, @code{symmetric}, @code{symplectic} and @code{lie_envelop}.

@opencatbox{Categories:}
@category{Package atensor}
@closecatbox

@end deffn

@anchor{adim}
@defvr {Variable} adim
Default value: 0

The dimensionality of the algebra. @code{atensor} uses the value of @code{adim}
to determine if an indexed object is a valid base vector.  See @code{abasep}.

@opencatbox{Categories:}
@category{Package atensor}
@category{Global variables}
@closecatbox

@end defvr

@anchor{aform}
@defvr {Variable} aform
Default value: @code{ident(3)}

Default values for the bilinear forms @code{sf}, @code{af}, and
@code{av}. The default is the identity matrix @code{ident(3)}.

@opencatbox{Categories:}
@category{Package atensor}
@category{Global variables}
@closecatbox

@end defvr

@anchor{asymbol}
@defvr {Variable} asymbol
Default value: @code{v}

The symbol for base vectors.

@opencatbox{Categories:}
@category{Package atensor}
@category{Global variables}
@closecatbox

@end defvr

@anchor{sf}
@deffn {Function} sf (@var{u}, @var{v})

A symmetric scalar function that is used in commutation relations.
The default implementation checks if both arguments are base vectors
using @code{abasep} and if that is the case, substitutes the
corresponding value from the matrix @code{aform}.

@opencatbox{Categories:}
@category{Package atensor}
@closecatbox

@end deffn

@anchor{af}
@deffn {Function} af (@var{u}, @var{v})

An antisymmetric scalar function that is used in commutation relations.
The default implementation checks if both arguments are base vectors
using @code{abasep} and if that is the case, substitutes the
corresponding value from the matrix @code{aform}.

@opencatbox{Categories:}
@category{Package atensor}
@closecatbox

@end deffn

@anchor{av}
@deffn {Function} av (@var{u}, @var{v})

An antisymmetric function that is used in commutation relations.
The default implementation checks if both arguments are base vectors
using @code{abasep} and if that is the case, substitutes the
corresponding value from the matrix @code{aform}.

For instance:

@c ===beg===
@c load("atensor");
@c adim:3;
@c aform:matrix([0,3,-2],[-3,0,1],[2,-1,0]);
@c asymbol:x;
@c av(x[1],x[2]);
@c ===end===
@example
(%i1) load("atensor");
(%o1)       /share/tensor/atensor.mac
(%i2) adim:3;
(%o2)                                  3
(%i3) aform:matrix([0,3,-2],[-3,0,1],[2,-1,0]);
                               [  0    3   - 2 ]
                               [               ]
(%o3)                          [ - 3   0    1  ]
                               [               ]
                               [  2   - 1   0  ]
(%i4) asymbol:x;
(%o4)                                  x
(%i5) av(x[1],x[2]);
(%o5)                                 x
                                       3
@end example

@opencatbox{Categories:}
@category{Package atensor}
@closecatbox

@end deffn


@anchor{abasep}
@deffn {Function} abasep (@var{v})

Checks if its argument is an @code{atensor} base vector. That is, if it is
an indexed symbol, with the symbol being the same as the value of
@code{asymbol}, and the index having a numeric value between 1
and @code{adim}.

@opencatbox{Categories:}
@category{Package atensor}
@category{Predicate functions}
@closecatbox

@end deffn
