Hello,

I am planning to use a symmetric tensor rank-6 for third-order elastic 
constants in a nonlinear elasticity code. The tensor is supposed to be 
symmetric within each three pair indices as well as all possible orders of 
pair permutations. 
Namely, C_ijklmn = C_jiklmn = C_ijlkmn = C_ijklnm = C_klijmn = C_mnijkl 
,.....This tensor without any material symmetry will have 56 independent 
elements. 

The SymmetricTensor class provides the symmetry within the pairs without 
the symmetry obtained by permutation of pairs and it will be totally fine 
and I can take care of the extra symmetry I need by writing the equal 
elements as many times as required.  

However, the problem is that I can not even initialize a rank-6 tensor. 
Interestingly it works for rank-4 though. It seems that I am missing 
something in the instruction of the tensor. Only the following simplest 
line of code gives me an error.

$ *SymmetricTensor<6, dim> C; *   


The error is:

/home/hbabaei/deal.ii-candi/deal.II-v9.0.1/include/deal.II/base/symmetric_tensor.h:563:29:
 
error: incomplete type 
‘dealii::internal::SymmetricTensorAccessors::StorageType<6, 3, double>’ 
used in nested name specifier
   static const unsigned int n_independent_components
                             ^
/home/hbabaei/deal.ii-candi/deal.II-v9.0.1/include/deal.II/base/symmetric_tensor.h:1046:1:
 
error: size of array is not an integral constant-expression
 SymmetricTensor<rank_,dim,Number>::SymmetricTensor (const Number (&array) 
[n_independent_components])
 ^
/home/hbabaei/deal.ii-candi/deal.II-v9.0.1/include/deal.II/base/symmetric_tensor.h:1046:100:
 
error: size of array is not an integral constant-expression
 SymmetricTensor<rank_,dim,Number>::SymmetricTensor (const Number (&array) 
[n_independent_components])
                                                                            
                        ^
/home/hbabaei/deal.ii-candi/deal.II-v9.0.1/include/deal.II/base/symmetric_tensor.h:861:61:
 
error: invalid use of incomplete type ‘struct 
dealii::internal::SymmetricTensorAccessors::StorageType<6, 3, double>’
   typedef typename base_tensor_descriptor::base_tensor_type 
base_tensor_type;
                                                             ^
/home/hbabaei/deal.ii-candi/deal.II-v9.0.1/include/deal.II/base/symmetric_tensor.h:188:12:
 
error: declaration of ‘struct 
dealii::internal::SymmetricTensorAccessors::StorageType<6, 3, double>’
     struct StorageType;
            ^

It seems that I need to specify n_independent_components, but I do not know 
how and where I can do so.

I appreciate it if you could give me any clue how to implement a rank-6 
tensor successfully.

Thanks and regards,

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