Update header.tex

Author: khanh101

assets/header.tex

@@ -82,6 +82,7 @@
 \DeclareMathOperator{\Cov}{Cov}
 \DeclareMathOperator{\RV}{RV}
 \DeclareMathOperator{\rank}{rank}
+\DeclareMathOperator{\ord}{ord}
 
 %% pair delimiter
 \DeclarePairedDelimiter{\abs}{\lvert}{\rvert}