# Curly brackets and Big O notation

I'm having some trouble inserting a Big O notation into a right-side curly bracket consisting of two lines. I'm using mathcal for the Big O notation but once I put it in the curly bracket it doesn't seem to work. I'm inserting an image of what I want to do (made in powerpoint). Very thankful for help :)

• Welcome to TeX.SE! Please post a minimal code of what you have tried so far. This kind of construction is usually best achieved with the cases environment from the amsmath package. Oct 7 '20 at 11:54
• Welcome to the TeX.SE: your question seem a bit contradictory I'm using mathcal for the Big O notation but once I put it in the curly bracket it doesn't seem to work. What is, please, the example that you have made using LaTeX? Oct 7 '20 at 11:56
• Thank you for the quick reply! I've added some images. As you can see, the Big-O notation on the second line is just a normal O when I put it in curly brackets Oct 7 '20 at 12:02
• Everything in the align environment is already in math mode and the two \$ are wrong: lose them. Oct 7 '20 at 12:03
• Thank you so much! :) Oct 7 '20 at 12:06

\documentclass[a4paper,12pt]{article}
$\begin{cases} T(1)=\mathcal{O}(1) & \\ T(m)=22\ast(T/2) \end{cases} \implies T(m)=22\ast(T/2)$