In my most recent post to Linear and Abstract Algebra I had a lot of trouble with subscripts

$\displaystyle N_1 $ was OK, but for some reason putting N_k or N_{k} in between $\displaystyle and $ did not work

I also wanted N to be subscipted by j-1 and j+1 but N_{j-1} and N_{j+1} did not work

Can anyone help?

Peter