Hi, I'm reading a book about fluid dynamics and I found some strange product between tensors. It's written like this: v=S:∇I , where S and I are matrices and v is a vector. Symbol : usually denotes Frobenius inner product. In this case we have a product of a matrix with a tensor of rank 3 and the result is vector - so it's not classical Frobenius product. I would like to know what kind of product this is (especially useful would be index notation). Thanks for help.