The sequence whose definition is: ``t is the first, fourth, eleventh, ... letter of this sentence.'' The first few values are 1, 4, 11, 16, 24, 29, 33, 35, 39, ... (Sloane's A005224).
References
Hofstadter, D. R. Metamagical Themas: Questing of Mind and Pattern. New York: BasicBooks, p. 44, 1985.
Sloane, N. J. A. Sequence
A005224/M3406
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.