amsmath + amssymb + mathtools — Math Typesetting

CTAN: https://ctan.org/pkg/amsmath | https://ctan.org/pkg/mathtools
Manual: texdoc amsmath, texdoc mathtools

Setup

\usepackage{amsmath}    % core math environments
\usepackage{amssymb}    % extra symbols (loads amsfonts)
\usepackage{mathtools}  % fixes + extensions for amsmath (loads amsmath)
\usepackage{amsthm}     % theorem environments

% mathtools loads amsmath, so you can just use:
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsthm}