help needed with a duplicate

[N. J. A. Sloane]

> it seems to me a(11) = -1 since 2^11 = 2048
> 
> What am I missing?
> 

Neil,
I agree a(11) = -1.  Here's what my Mathematica code produces:

{-1, -1, -1, -1, -1, -1, 1, -1, -1, -1, 1, -1, -1, -1, -1, -1, -1, \
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, \
-1, 3, 10, 2, -1, -1, -1, -1, -1, -1, 10, -1, -1, -1, -1, 5, -1, 3, \
-1, -1, -1, -1, -1, -1, -1, -1, 11, -1, -1, -1, -1, -1, -1, 13, -1, \
-1, -1, -1, 2, 5, -1, -1, -1, -1, -1, -1, -1, -1, 20, -1, -1, 4, -1, \
-1, -1, 3, -1, -1, -1, 24, -1, -1, -1, -1, -1, -1}

Whatever it is you are missing, I'm missing too.
Ray