Hofstadter H-Sequence
The sequence defined by 
and
The first few terms are 1, 1, 2, 3, 4, 4, 5, 5, 6, 7, 7, 8, 9, 10, 10, 11, 12, 13, 13, 14, ... (Sloane's A005374).
References
Hofstadter, D. R. Gödel, Escher, Bach: An Eternal Golden Braid. New York: Vintage Books, p. 137, 1989. Sloane, N. J. A. SequenceA005374/M0449in ``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.
- Jordan's Lemma
- Josephus Problem
- Jug
- Jugendtraum
- Juggling
- Julia Fractal
- Julia Set
- Jump
- Jump Angle
- Jumping Champion
- Jung's Theorem
- Just If
- Just One
- Kabon Triangles
- Kac Formula
- Kac Matrix
- Kakeya Needle Problem
- Kakutani's Fixed Point Theorem
- Kakutani's Problem
- Kalman Filter
- Kampyle of Eudoxus
- KAM Theorem
- Kanizsa Triangle
- Kantorovich Inequality
- Kaplan-Yorke Conjecture