# Error for more or less standard variational formulation using two cross products

**URL:** <https://fenicsproject.discourse.group/t/error-for-more-or-less-standard-variational-formulation-using-two-cross-products/5486>\
**Category:** variational formulation\
**Created:** [April 1, 2021, 11:26am UTC](https://fenicsproject.discourse.group/t/error-for-more-or-less-standard-variational-formulation-using-two-cross-products/5486 "2021-04-01T11:26:59Z")\
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**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\
**Post date:** [April 1, 2021, 12:15pm UTC](https://fenicsproject.discourse.group/t/error-for-more-or-less-standard-variational-formulation-using-two-cross-products/5486/4 "2021-04-01T12:15:46Z")

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From the ufl documentation: [Form language — Unified Form Language (UFL) 2019.2.0.dev0 documentation](https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html?highlight=Cross#cross)

The operator `cross` accepts as arguments two logically vector-valued expressions and returns a vector which is the cross product (vector product) of the two vectors:

𝚌𝚛𝚘𝚜𝚜(𝚟,𝚠)↔v×w=(v1w2−v2w1,v2w0−v0w2,v0w1−v1w0)

Note that this operator is only defined for vectors of length three.

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