A number of the form obtained by setting in a Fermat-Lucas Polynomial. The first few are 3, 5, 9, 17, 33, ... (Sloane's A000051).
See also Fermat Number (Lucas)
Sloane, N. J. A. Sequence
in ``An On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html and Sloane, N. J. A. and Plouffe, S.
The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.