How to add superscript on summation symbol

Question

I'd like to add superscript on summation symbol, but I get an error message:

Please, use `\mathaccent` for accents in math mode. 
I can't use ^ symbol to add superscript.

Here is the code:

\begin{equation}
    y & = \beta_{0}+\sumˆ{2}_{j=1}\beta_{j}X_{j}+\sum\sum_{i<j}\beta_{ij}X_{i}X{j}+\sum_{j=1}\beta_{jj}X_{j}\textsuperscript{2}
    \label{equation1}
\end{equation}

Answer 1

You have a wrong ^ character that is not a ascii character but possibly something with similar appearance and this is what causes the whole problem.

Your code with the correct character is something like:

\documentclass{article}

\begin{document}
\begin{equation}
 y = \beta_{0}+\sum_{j=1}^{2} \beta_{j}X_{j}+\sum\sum_{i<j}\beta_{ij}X_{i}X{j}+\sum_{j=1}^2\beta_{jj}X_{j}^2
    \label{equation1}
\end{equation}
\end{document}

And gives:

PS1: Not sure about the last power and the last upper limit of the sum, but you will see it works if you just delete your bad character and replace with the original keyboard's ^.

PS2: For readability never start a sum by its upper limit, but use first its lower limit instead.