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documentation:guide:symmetries_of_tensors [2015/11/21 12:33] (current)
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 +====== Symmetries of tensors ======
 +<​html>​
 +<div class="​text-right"​ style="​font-size:​ 15px; ">
 +</​html>​
 +Next topic: [[documentation:​guide:​Representation of derivatives]]
 +<​html>​
 +<span class="​glyphicon glyphicon-arrow-right"></​span>​
 +</​div>​
 +</​html>​
 +
 +----
 +
 +====Specifying symmetries of tensors====
 +One of the distinctive features of tensors is the presence of symmetries. Consider symmetries under permutations of indices. Permutational symmetries in Redberry can be defined for indices of [[documentation:​ref:​SimpleTensor]] and [[documentation:​ref:​TensorField]]. ​
 +
 +Let's for example define symmetries of Riemann tensor. All symmetries of Riemann tensor are:
 +\[
 +R_{abcd} = R_{cdab} = - R_{bacd} = -R_{abdc} = -R_{dcab} = -R_{cdba} = R_{dcba} = R_{badc}
 +\]
 +Only first two symmetries can be used as generators of the corresponding [[documentation:​ref:​PermutationGroup]]. The first one can be written as ''​(2,​3,​0,​1)''​ in one-line notation or as ''​(0,​2)(1,​3)''​ in cycle notation. Redberry allows to input symmetries both in one-line and cycle notation. So, symmetries of e.g. Riemann tensor in Redberry can be set up in the following way:
 +<sxh groovy; gutter: false>
 +//add first symmetry in cycle notation
 +addSymmetry '​R_abcd', ​ [[0, 2], [1, 3]].p 
 +//add second (anti)symmetry in one-line notation
 +addSymmetry '​R_abcd',​ -[1, 0, 2, 3].p 
 +</​sxh>​
 +Method ''​addSymmetry''​ have two arguments: a simple tensor (or its  string representation) and a [[documentation:​ref:​Permutation]]. Redberry has internal representation of [[documentation:​guide:​permutations_and_permutation_groups|permutations and permutation groups]]. In order to convert array to permutation one can use ''​.p''​ property followed after the array written in one-line or disjoint cycles notation. Minus (used in the last line) converts symmetry to 
 +antisymmetry and vice versa. ​
 +
 +Once set, symmetries of tensor affect all further manipulations with it. For example, if Riemann symmetries are set up, then the following code automatically gives zero:
 +<sxh groovy; gutter: false>
 +println '​R^abcd*R_efdc*R^ef_ab + R_rc^df*R_ab^rc*R_fd^ba'​.t
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > 0
 +</​sxh>​
 +Here zero was returned right after parsing; this is because Redberry automatically reduced sum to the [[documentation:​guide:​standard_form_of_mathematical_expressions|standard form]] and 
 +\[
 +R^{abcd} R_{efdc} R^{ef}{}_{ab} = -R_{rc}{}^{df} R_{ab}{}^{rc} R_{fd}{}^{ba}
 +\]
 +according to the specified symmetries. Such architecture requires user to set all symmetries of [[documentation:​ref:​SimpleTensor]] before it will be parsed inside any complicated ​ structure like [[documentation:​ref:​Sum]] or [[documentation:​ref:​Product]]. If one try to add symmetry to tensor which is already in use in some complicated expression, then the exception will be thrown.
 +
 +In case of tensors with multiple types of indices it becomes useful to specify symmetry just for indices of particular [[documentation:​ref:​IndexType|type]]:​
 +<sxh groovy; gutter: false>
 +addSymmetry 'f_{a b \\mu \\nu}'​.t,​ GreekLower, -[1, 0].p
 +println 'f_{m n \\alpha \\beta} + f_{m n \\beta \\alpha}'​.t
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > 0
 +</​sxh>​
 +====Internal handling of permutation groups====
 +Internally, Redberry aggregates symmetries of [[documentation:​ref:​SimpleTensor]] in a special container which can be accessed using ''​.symmetries''​ property of tensor indices. When all generators are specified, Redberry uses internal representation of [[documentation:​ref:​PermutationGroup]] to hold and manipulate symmetries. The following lines highlights some features of permutation groups of tensor indices:
 +<sxh groovy; gutter: true>
 +//set up symmetries of Riemann tensor
 +addSymmetry '​R_abcd', ​ [[0, 2], [1, 3]].p
 +addSymmetry '​R_abcd',​ -[1, 0, 2, 3].p
 +
 +def t = '​R_abcd'​.t ​
 +//container of generators
 +def symmetries = t.indices.symmetries
 +//​permutation group of Riemann tensor
 +def group = symmetries.permutationGroup
 +//total number of all permutations
 +println group.order()
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > 8
 +</​sxh>​
 +<sxh groovy; gutter: true; first-line: 12>
 +//compute setwise stabilizer of set [2, 3]
 +println ​ g.setwiseStabilizer(2,​ 3) 
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > Group( -[[0, 1]], -[[2, 3]] )
 +</​sxh>​
 +<sxh groovy; gutter: true; first-line: 14>
 +//define some other permutation group
 +def oth = Group(-[[0, 2, 1, 3]], -[[0, 1]], [[2, 3, 4, 5, 6]])
 +//compute intersections of groups
 +println g.intersection(oth) ​
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > Group( -[[2, 3]], +[[0, 2], [1, 3]] )
 +</​sxh>​
 +
 +One should be careful when attaching antisymmetries. Consider the following code:
 +<sxh groovy; gutter: false>
 +def t = '​R_abcd'​.t
 +addSymmetries t, [2, 3, 0, 1].p, -[1, 0, 2, 3].p, -[3, 2, 1, 0].p
 +def gr =  t.indices.symmetries.permutationGroup
 +println gr.order()
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > InconsistentGeneratorsException
 +</​sxh>​
 +Exception is thrown on the last line since the last attached [[documentation:​ref:​permutation]] is a combination of two previous, but its sign is different (i.e. it is antisymmetry,​ while to previous are symmetries). Thus, some combinations of symmetries and antisymmetries can come into conflict, which causes an exception.
 +====Finding symmetries of complicated tensors====
 +Redberry provides tools to find permutational symmetries of complicated tensors.
 +Consider the following example:
 +<sxh groovy; gutter: true>
 +addSymmetry '​R_abc',​ -[1, 0, 2].p
 +addSymmetry '​A_ab',​ [1, 0].p
 +def t = '​(R_abc*A_de + R_bde*A_ac)*A^ce + R_adb'​.t
 +def symmetries = findIndicesSymmetries('​_abd'​.si,​ t)
 +for (s in symmetries)
 +    println s
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > +[[]]
 +   > -[[0, 2]]
 +</​sxh>​
 +The first [[documentation:​ref:​permutation]] is identity, while the second is nontrivial. Method used in the fourth line takes [[documentation:​ref:​SimpleIndices]] as the first argument in order to define the relative order of indices in tensor ('''​...'​.si''​ construction is used to parse [[documentation:​ref:​simpleindices]]). ​
 +====Multi-term symmetries====
 +The so-called multi-terms symmetries (like Bianchi identities) are faintly covered or absent at all in the majority of existing systems. In fact, we know only one system --- [[http://​cadabra.phi-sci.com|Cadabra]] which fully supports multi-term symmetries. The basic idea utilized by Cadabra system is usage of Young tableau projectors to reduce expressions to the simplified form. Consider the following identity (this example is taken from Section 2.2 of the //Cadabra: reference guide and tutorial// by Kasper Peeters, which is available on Cadabra web site):
 +\begin{multline}
 +W_{u}{}^{vs}{}_{w} W_{tv}{}^{qw} ​
 +W_{p}{}^{t}{}_{r}{}^{u} W^{p}{}_{q}{}^{r}{}_{s}
 +-
 +W^{sv}{}_{u}{}^{w} W_{rvtw} ​
 +W_{p}{}^{qtu} W^{p}{}_{q}{}^{r}{}_{s}
 += \\ =
 +W_{s}{}^{pd}{}_{a} W^{ms}{}_{cd} ​
 +W^{n}{}_{pb}{}^{c} W_{mn}{}^{ab}
 +-
 +\frac{1}{4} W^{n}{}_{s}{}^{d}{}_{c} W^{m}{}_{p}{}^{c}{}_{d} ​
 +W^{ps}{}_{ba} W_{mn}{}^{ab}
 +\end{multline}
 +where $W_{abcd}$ is a Weyl tensor. In order to proof this identity using a Young projector, one need to apply the following substitution to the above expression:
 +\begin{equation}
 +\label{eq:​WeylYoung}
 +W_{abcd} = \frac{1}{3} \, (2\,​W_{abcd}-W_{adbc}+W_{acbd})
 +\end{equation}
 +This identity is derived from the Ricci cyclic identity and ordinary permutational symmetries of the Weyl tensor. The following Redberry code proofs the identity:
 +<sxh groovy; gutter: false>
 +addSymmetries '​W_abcd',​ [[0, 2], [1, 3]].p, -[1, 0, 2, 3].p
 +def t = ('​W^p_q^r_s*W_p^t_r^u*W_tv^qw*W_u^vs_w'​
 +        + ' - W^p_q^r_s*W_p^qtu*W_rvtw*W^sv_u^w'​
 +        + ' - W_mn^ab*W^n_pb^c*W^ms_cd*W_s^pd_a'​
 +        + ' + 1/​4*W_mn^ab*W^ps_ba*W^m_p^c_d*W^n_s^d_c'​).t
 +def s = '​W_mnpq = 1/​3*(2*W_mnpq - W_mqnp + W_mpnq)'​.t
 +def r = (s & Expand) >> t
 +println r
 +</​sxh>​
 +<sxh plain; gutter: false>
 +   > 0
 +</​sxh>​
 +At the moment Redberry has no built-in functionality to construct such substitutions based on the Young projectors. But, as could be seen from the above example, if one define the corresponding substitution manually, then Redberry allows to work with multi-term symmetries in the Cadabra way. The support of Young projectors is planned in the upcoming releases of Redberry.
 +
 +====See also====
 +  * Related guides: [[documentation:​guide:​permutations_and_permutation_groups]]
 +  * Related reference material: [[documentation:​ref:​simpleindices]],​ [[documentation:​ref:​permutation]],​ [[documentation:​ref:​permutationgroup]]