Aliquot sequence
Unsolved problem in mathematics: Do all aliquot sequences eventually end with a prime number, a perfect number, or a set of amicable or sociable numbers? (Catalan's aliquot sequence conjecture) (more unsolved problems in mathematics)

In mathematics, an aliquot sequence is a sequence of positive integers in which each term is the sum of the proper divisors of the previous term. If the sequence reaches the number 1, it ends, since the sum of the proper divisors of 1 is 0.
Definition and overview
The aliquot sequence starting with a positive integer k can be defined formally in terms of the sumofdivisors function σ_{1} or the aliquot sum function s in the following way:^{[1]}
 s_{0} = k
 s_{n} = s(s_{n−1}) = σ_{1}(s_{n−1}) − s_{n−1} if s_{n−1} > 0
 s_{n} = 0 if s_{n−1} = 0 > (if we add this condition, then the terms after 0 are all 0, and all aliquot sequences would be infinite sequence, and we can conjecture that all aliquot sequences are convergent, the limit of these sequences are usually 0 or 6)
and s(0) is undefined.
For example, the aliquot sequence of 10 is 10, 8, 7, 1, 0 because:
 σ_{1}(10) − 10 = 5 + 2 + 1 = 8,
 σ_{1}(8) − 8 = 4 + 2 + 1 = 7,
 σ_{1}(7) − 7 = 1,
 σ_{1}(1) − 1 = 0.
Many aliquot sequences terminate at zero; all such sequences necessarily end with a prime number followed by 1 (since the only proper divisor of a prime is 1), followed by 0 (since 1 has no proper divisors). See (sequence A080907 in the OEIS) for a list of such numbers up to 75. There are a variety of ways in which an aliquot sequence might not terminate:
 A perfect number has a repeating aliquot sequence of period 1. The aliquot sequence of 6, for example, is 6, 6, 6, 6, ...
 An amicable number has a repeating aliquot sequence of period 2. For instance, the aliquot sequence of 220 is 220, 284, 220, 284, ...
 A sociable number has a repeating aliquot sequence of period 3 or greater. (Sometimes the term sociable number is used to encompass amicable numbers as well.) For instance, the aliquot sequence of 1264460 is 1264460, 1547860, 1727636, 1305184, 1264460, ...
 Some numbers have an aliquot sequence which is eventually periodic, but the number itself is not perfect, amicable, or sociable. For instance, the aliquot sequence of 95 is 95, 25, 6, 6, 6, 6, ... . Numbers like 95 that are not perfect, but have an eventually repeating aliquot sequence of period 1 are called aspiring numbers.^{[2]}
n  Aliquot sequence of n  length (OEIS: A098007)  n  Aliquot sequence of n  length (OEIS: A098007)  n  Aliquot sequence of n  length (OEIS: A098007)  n  Aliquot sequence of n  length (OEIS: A098007) 
0  0  1  12  12, 16, 15, 9, 4, 3, 1, 0  8  24  24, 36, 55, 17, 1, 0  6  36  36, 55, 17, 1, 0  5 
1  1, 0  2  13  13, 1, 0  3  25  25, 6  2  37  37, 1, 0  3 
2  2, 1, 0  3  14  14, 10, 8, 7, 1, 0  6  26  26, 16, 15, 9, 4, 3, 1, 0  8  38  38, 22, 14, 10, 8, 7, 1, 0  8 
3  3, 1, 0  3  15  15, 9, 4, 3, 1, 0  6  27  27, 13, 1, 0  4  39  39, 17, 1, 0  4 
4  4, 3, 1, 0  4  16  16, 15, 9, 4, 3, 1, 0  7  28  28  1  40  40, 50, 43, 1, 0  5 
5  5, 1, 0  3  17  17, 1, 0  3  29  29, 1, 0  3  41  41, 1, 0  3 
6  6  1  18  18, 21, 11, 1, 0  5  30  30, 42, 54, 66, 78, 90, 144, 259, 45, 33, 15, 9, 4, 3, 1, 0  16  42  42, 54, 66, 78, 90, 144, 259, 45, 33, 15, 9, 4, 3, 1, 0  15 
7  7, 1, 0  3  19  19, 1, 0  3  31  31, 1, 0  3  43  43, 1, 0  3 
8  8, 7, 1, 0  4  20  20, 22, 14, 10, 8, 7, 1, 0  8  32  32, 31, 1, 0  4  44  44, 40, 50, 43, 1, 0  6 
9  9, 4, 3, 1, 0  5  21  21, 11, 1, 0  4  33  33, 15, 9, 4, 3, 1, 0  7  45  45, 33, 15, 9, 4, 3, 1, 0  8 
10  10, 8, 7, 1, 0  5  22  22, 14, 10, 8, 7, 1, 0  7  34  34, 20, 22, 14, 10, 8, 7, 1, 0  9  46  46, 26, 16, 15, 9, 4, 3, 1, 0  9 
11  11, 1, 0  3  23  23, 1, 0  3  35  35, 13, 1, 0  4  47  47, 1, 0  3 
The lengths of the aliquot sequences that start at n are
 1, 2, 2, 3, 2, 1, 2, 3, 4, 4, 2, 7, 2, 5, 5, 6, 2, 4, 2, 7, 3, 6, 2, 5, 1, 7, 3, 1, 2, 15, 2, 3, 6, 8, 3, 4, 2, 7, 3, 4, 2, 14, 2, 5, 7, 8, 2, 6, 4, 3, ... (sequence A044050 in the OEIS)
The final terms (excluding 1) of the aliquot sequences that start at n are
 1, 2, 3, 3, 5, 6, 7, 7, 3, 7, 11, 3, 13, 7, 3, 3, 17, 11, 19, 7, 11, 7, 23, 17, 6, 3, 13, 28, 29, 3, 31, 31, 3, 7, 13, 17, 37, 7, 17, 43, 41, 3, 43, 43, 3, 3, 47, 41, 7, 43, ... (sequence A115350 in the OEIS)
Numbers whose aliquot sequence terminates in 1 are
 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, ... (sequence A080907 in the OEIS)
Numbers whose aliquot sequence known to terminate in a perfect number, other than perfect numbers themselves (6, 28, 496, ...), are
 25, 95, 119, 143, 417, 445, 565, 608, 650, 652, 675, 685, 783, 790, 909, 913, ... (sequence A063769 in the OEIS)
Numbers whose aliquot sequence terminates in a cycle with length at least 2 are
 220, 284, 562, 1064, 1184, 1188, 1210, 1308, 1336, 1380, 1420, 1490, 1604, 1690, 1692, 1772, 1816, 1898, 2008, 2122, 2152, 2172, 2362, ... (sequence A121507 in the OEIS)
Numbers whose aliquot sequence is not known to be finite or eventually periodic are
 276, 306, 396, 552, 564, 660, 696, 780, 828, 888, 966, 996, 1074, 1086, 1098, 1104, 1134, 1218, 1302, 1314, 1320, 1338, 1350, 1356, 1392, 1398, 1410, 1464, 1476, 1488, ... (sequence A131884 in the OEIS)
A number that is never the successor in an aliquot sequence is called an untouchable number.
 2, 5, 52, 88, 96, 120, 124, 146, 162, 188, 206, 210, 216, 238, 246, 248, 262, 268, 276, 288, 290, 292, 304, 306, 322, 324, 326, 336, 342, 372, 406, 408, 426, 430, 448, 472, 474, 498, ... (sequence A005114 in the OEIS)
Catalan–Dickson conjecture
An important conjecture due to Catalan, sometimes called the Catalan–Dickson conjecture, is that every aliquot sequence ends in one of the above ways: with a prime number, a perfect number, or a set of amicable or sociable numbers.^{[3]} The alternative would be that a number exists whose aliquot sequence is infinite yet never repeats. Any one of the many numbers whose aliquot sequences have not been fully determined might be such a number. The first five candidate numbers are often called the Lehmer five (named after D.H. Lehmer): 276, 552, 564, 660, and 966.^{[4]} However, it is worth noting that 276 may reach a high apex in its aliquot sequence and then descend; the number 138 reaches a peak of 179931895322 before returning to 1.
Guy and Selfridge believe the Catalan–Dickson conjecture is false (so they conjecture some aliquot sequences are unbounded above (i.e., diverge)).^{[5]}
(As of April 2015), there were 898 positive integers less than 100,000 whose aliquot sequences have not been fully determined, and 9190 such integers less than 1,000,000.^{[6]}
Systematically searching for aliquot sequences
The aliquot sequence can be represented as a directed graph, [math]\displaystyle{ G_{n,s} }[/math], for a given integer [math]\displaystyle{ n }[/math], where [math]\displaystyle{ s(k) }[/math] denotes the sum of the proper divisors of [math]\displaystyle{ k }[/math].^{[7]} Cycles in [math]\displaystyle{ G_{n,s} }[/math] represent sociable numbers within the interval [math]\displaystyle{ [1,n] }[/math]. Two special cases are loops that represent perfect numbers and cycles of length two that represent amicable pairs.
See also
Notes
 ↑ Weisstein, Eric W.. "Aliquot Sequence". http://mathworld.wolfram.com/AliquotSequence.html.
 ↑ Sloane, N. J. A., ed. "Sequence A063769 (Aspiring numbers: numbers whose aliquot sequence terminates in a perfect number.)". OEIS Foundation. https://oeis.org/A063769.
 ↑ Weisstein, Eric W.. "Catalan's Aliquot Sequence Conjecture". http://mathworld.wolfram.com/CatalansAliquotSequenceConjecture.html.
 ↑ Creyaufmüller, Wolfgang (May 24, 2014). "Lehmer Five". http://www.aliquot.de/lehmer.htm.
 ↑ A. S. Mosunov, What do we know about aliquot sequences?
 ↑ Creyaufmüller, Wolfgang (April 29, 2015). "Aliquot Pages". http://www.aliquot.de/aliquote.htm.
 ↑ Rocha, Rodrigo Caetano; Thatte, Bhalchandra (2015), Distributed cycle detection in largescale sparse graphs, Simpósio Brasileiro de Pesquisa Operacional (SBPO), doi:10.13140/RG.2.1.1233.8640
References
 Manuel Benito; Wolfgang Creyaufmüller; Juan Luis Varona; Paul Zimmermann. Aliquot Sequence 3630 Ends After Reaching 100 Digits. Experimental Mathematics, vol. 11, num. 2, Natick, MA, 2002, p. 201–206.
 W. Creyaufmüller. Primzahlfamilien  Das Catalan'sche Problem und die Familien der Primzahlen im Bereich 1 bis 3000 im Detail. Stuttgart 2000 (3rd ed.), 327p.
External links
 Current status of aliquot sequences with start term below 2 million
 Tables of Aliquot Cycles (J.O.M. Pedersen)
 Aliquot Page (Wolfgang Creyaufmüller)
 Aliquot sequences (Christophe Clavier)
 Forum on calculating aliquot sequences (MersenneForum)
 Aliquot sequence summary page for sequences up to 100000 (there are similar pages for higher ranges) (Karsten Bonath)
 Active research site on aliquot sequences (JeanLuc Garambois) (in French)
Original source: https://en.wikipedia.org/wiki/Aliquot sequence.
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