All good answers. I would just like to mention that if your are somewhat new to LaTeX, you should get used to using \newcommand and \renewcommand. For example:
% __________ Differentials __________
% Single
\newcommand{\diff}{d} % If you want an upright `d', change it here \newcommand{\p}[1]{\partial#1} \newcommand{\q}[1]{\delta#1} % Kronecker Delta/Variation Symbol
% Nth Differentials:
\newcommand{\dN}[2]{\diff^{#1}{#2}} % Numerator type \newcommand{\pN}[2]{\partial^{#1}{#2}} \newcommand{\qN}[2]{\delta^{#1}{#2}}
% Nth Powers of Differentials
\newcommand{\dD}[2]{\diff{#2}^{#1}} % Denominator type \newcommand{\pD}[2]{\p{#2}^{#1}} \newcommand{\qD}[2]{\delta{#2}^{#1}} \newcommand{\pd}[1]{\dfrac{\partial}{\partial{#1}}} \newcommand{\pdd}[2]{\dfrac{\partial{#1}}{\partial{#2}}}
% __________ Derivatives __________
% 1st derivative:
\newcommand{\dod}[2]{\dfrac{\diff{#1}}{\diff{#2}}} % 'differential over differential' \newcommand{\pop}[2]{\dfrac{\p#1}{\p#2}} % 'partial over partial' \newcommand{\lpop}[2]{\p#1/\p#2} % A 'layed down' version \newcommand{\qoq}[2]{\dfrac{\q#1}{\q#2}}
% Nth derivative:
\newcommand{\dodN}[3]{\dfrac{\dN{#1}{#2}}{\dD{#1}{#3}}} \newcommand{\popN}[3]{\dfrac{\pN{#1}{#2}}{\pD{#1}{#3}}} \newcommand{\lpopN}[3]{\pN{#1}{#2}/\pD{#1}{#3}} % Layed version of \pop \newcommand{\qoqN}[3]{\dfrac{\qN{#1}{#2}}{\qD{#1}{#3}}}
% Mixed
\newcommand{\dodMixed}[3]{\dfrac{\dN{2}{#1}}{\diff{#2}\diff{#3}}} \newcommand{\popMixed}[3]{\dfrac{\pN{2}{#1}}{\p{#2}\p{#3}}} \newcommand{\lpopMixed}[3]{\pN{2}{#1}/\p{#2}\p{#3}} \newcommand{\qoqMixed}[3]{\dfrac{\qN{2}{#1}}{\q{#2}\q{#3}}}
cool: "The package (COntent Oriented LaTeX) gives LaTeX the power to retain mathematical meaning of its expressions in addition to the typsetting instructions; essentially separating style from the content of the math."