# Miscellaneous tricks and tips

## LINUX commands

`fs setacl . system:anyuser read`

allow anyone to read current directory

## Matrix calculus

- If
*A*, *B*, and *C* are square matrices, then tr *ABC* = tr *BCA* = tr *CAB*.
- ∇
_{A} tr *AB* = *B*^{T}
- ∇
_{AT} *f*(*A*) = (∇_{A} *f*(*A*))^{T}
- ∇
_{A}tr *ABA*^{T}*C* = *CAB* + *C*^{T}*AB*^{T}
- ∇
_{A}|*A*| = |*A*|(*A*^{-1})^{T} if *A* is a non-singular square matrix.

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