How do I get LaTeX to put the infinity on top of the SUM symboL

[laura_a]

I need to make a power series in LaTex that looks like this
u = \Sigma^{\infty}_{k=0} a_kx^k

But I wan the infinity on top of the sum and the k=0 on the bottom like normal setting out but I can't find out how to do it?
Hopefully someone from here can show me! Thanks

 

[HallsofIvy]

Use "sum" instead of "Sigma":

u= \sum_{k=0}^\infty a_kx^k

 

[laura_a]

thanks!