Prime Factors
The number of Distinct Prime Factors of a number 
is denoted 
. The first few values for 
,2, ... are 0, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 1, 2, 2, 1, 1, 2, 1, 2, ... (Sloane's A001221; top figure). Thenumber of not necessarily distinct prime factors of a number is denoted 
. The first few values for ,2, ... are 0, 1, 1, 2, 1, 2, 1, 3, 2, 2, 1, 3, 1, 2, 2, 4, 1, 3, 1, 3, ... (Sloane's A001222; bottom figure).
References
Sloane, N. J. A. SequencesA001222/M0094and A001221/M0056in ``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.
- Irregular Singularity
- Irrotational Field
- Isarithm
- ISBN
- I-Signature
- Island
- Isobaric Polynomial
- Isochronous Curve
- Isoclinal
- Isocline
- Isoclinic Groups
- Isodynamic Points
- Isoenergetic Nondegeneracy
- Isogonal Conjugate
- Isogonal Line
- Isogonic Centers
- Isograph
- Isohedral Tiling
- Isohedron
- Isolated Point
- Isolated Singularity
- Isolating Integral
- Isometric Latitude
- Isometry
- Isomorphic Graphs