Perhaps, this is interesting (?)

[creigh at o2online.de]

My apologies...
my statement was merely a trivial consequence of the formula
given (thanks to Isabel):
*******
 From the Encyclopedia listing for A087289: 

a(n) = 2^(2*n+1) + 1. 

your formula follows. 

-Isabel 
*********

Sincerely, 
Creighton