Tensor index problem: Repeated Uncaught Throw[Null] error messages

[HL D]

Dear all,

I am absolutely new to xAct - I just installed the package yesterday on my Mathematica 8 on Mac OS (High Sierra), and I am learning the syntax of xAct by following the basic examples (line by line) posted on the xAct package site:

http://www.xact.es/Documentation/HTML/HTMLLinks/xTensorDoc.nb_3.html

 The examples are very basic, but I encountered 2 error messages of the kind:

Throw::nocatch :  

 in place of the expected straightforward result shown in the example.

1. The first example I tried to replicate (and failed to get the expected result) is

In[6]:= F[-a, -b] v[d] v[b] v[-b] + 3 v[b] v[c] v[-c] F[-b, d] F[-d, -a]

In[7]:= % // Simplification

In[7]:= Validate::repeated :  "Found indices with the same name "-b"."
In[7]:= Throw::nocatch :  "Uncaught Throw[Null] returned to top level. 
Out[7]= Hold[Throw[Null]]

2. The second example involves the DefCovD (covariant derivative definition):  DefCovD[Cd[-a], {";", "\[Del]"}], which automatically involves the definition of the Riemann tensor Riemann[-a,-b,-c,d] that is antisymmetric in a,b. However, when I run:

RiemannCd[-a, -b, -c, d] + RiemannCd[-b, -a, -c, -d] // Simplification

which should yield the zero 0, I get the same type of error message:

In[11]:= Validate::inhom :  "Found inhomogeneous "indices": "{IndexList[-a, -b, -c, -d], IndexList[-a, -b, -c, d]}"."
In[11]:= Throw::nocatch :  "Uncaught Throw[Null] returned to top level. 
Out[11]= Hold[Throw[Null]]

I don't know what went wrong with my xAct package - all other results in these basic examples evaluate correctly, except for those involving upper indices. 

Please help !

Many thanks,

HL

[Pedram Karimi]

Dear HL,

xAct is extremely sensitive to the position of indices, in your first example you repeated the upper (contravariant) index "b". I believe the true expression is 

 F[-a, -d] v[d] v[b] v[-b] + 3 v[b] v[c] v[-c] F[-b, d] F[-d, -a]

Your second example has the same kind of error you can fix it as follow

RiemannCd[-a, -b, -c, d] + RiemannCd[-b, -a, -c, d] // Simplification

best,

Pedram

[HL D]

Dear Pedram,

Thanks sooo much ! Your suggestions worked marvellously - I got the expected results!!!

 I did this tutorial at 2am in the morning so I didn't notice the index mistakes that I made. 

Thanks again !!!

Best,

HL