Given the expression
$s\in\{(u_0,u_1)\in \Bbb R \times\Bbb R^3\},$
what is s?
- a single vector in $\Bbb R^4$ consisting a concatenation of the elements of $u_0\in \Bbb R$ and $u_1\in \Bbb R^3$
- a set of two vectors in $\{u_0, u_1\in\Bbb R^4\}$
- a subset consisting of a element $u_0\in\Bbb R$ and an element $u_1\in\Bbb R^3$
- a matrix in $\Bbb R^{u_0\times3}$
Also, how would the following be expressed: "s is either a real number or a 3-vector given that the real number equals the norm of the 3-vector"
Is this along the right lines? $s\in\{u_0\in\Bbb R, u_1\in\Bbb R^3\,|\,u_0=||u_1||_2\}$