# Summations of a summation without upper bound as a lemma

So I have a paper I'm writing for a class that requires me to include a proof. I have the proof all sorted out on paper but I'm having a very hard time figuring out where to begin in Latex. I need to include the picture below as a lemma so it will appear in the equation listed in the code below. I already have \newtheorem{lemm}{Lemma} at the top of my document. Basically, if I can figure out how to code the equation from the image into Latex as a lemma, I think I'll have everything I need to code the rest of the proof. I'm very new at Latex so this is a daunting task for me. Any feedback or suggestions would be much appreciated.

\begin{lemm}
A(x)B(x)=
\end{lemm}


• I would recommend that you find a (not so) gentle introduction to LaTeX. This example wouldn't have been too hard to do from that. – Teepeemm Apr 24 '19 at 21:56

Something like this?

\documentclass{article}
\usepackage{amsthm}
\newtheorem{lemm}{Lemma}
\begin{document}
\begin{lemm}
$A(x)B(x) = \sum_{k\ge0} \biggl( \sum_{i=0}^{k}a_ib_{k-i} \biggr) x^k$
\end{lemm}

\end{document}

• You are a life saver! Thank you very much! I think I should be able to finish the proof from here. – Alex Adinolfi Apr 24 '19 at 21:42