Practical Number
A number 
is practical if for all 
, 
is the sum of distinct proper divisors of . Defined in 1948 byA. K. Srinivasen. All even Perfect Numbers are practical. The number
is practical for all
, 3, .... The first few practical numbers are 1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 28, 30, 32,36, 40, 42, 48, 54, 56, ... (Sloane's A005153). G. Melfi has computed twins, triplets, and 5-tuples of practical numbers.The first few 5-tuples are 12, 18, 30, 198, 306, 462, 1482, 2550, 4422, ....
References
Melfi, G. ``On Two Conjectures About Practical Numbers.'' J. Number Th. 56, 205-210, 1996. Melfi, G. ``Practical Numbers.'' http://www.dm.unipi.it/gauss-pages/melfi/public_html/pratica.html. Sloane, N. J. A. SequenceA005153/M0991in ``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.
- Riemannian Metric
- Riemann Integral
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- Riemann-Mangoldt Function
- Riemann Mapping Theorem
- Riemann P-Differential Equation
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- Riemann's Moduli Problem
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- Riesel Number