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In mathematics, an aliquot sequence is a recursive sequence in which each term is the sum of the proper divisors of the previous term. The aliquot sequence starting with an integer k can be defined formally in terms of the sum-of-divisors function σ in the following way:

s₀ = k

s_n = σ(s_n−1)

For example, the aliquot sequence of 10 is 10, 8, 7, 1, 0.

Many aliquot sequences terminate at zero (sequence A080907 in OEIS); 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). 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 (A000396). The aliquot sequence of 6, for example, is 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, .... Numbers that like 95 are not perfect but have a repeating aliquot sequence of period 1 are called aspiring numbers (A063769).

An important conjecture due to Catalan with respect to aliquot sequences 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. The alternative would be that a number exists whose aliquot sequence is infinite, yet aperiodic. There are several numbers whose aliquot sequences as of 2006 have not been fully determined, and thus might be such a number. The first five candidate numbers are called the "Lehmer Five": 276, 552, 564, 660, and 966.

As of July 2005, there are 913 numbers in [1, 10⁵] whose aliquot sequences have not been fully determined, and 9474 such numbers in [1, 10⁶].

• Aliquot
• Aliquant

• Aliquot Pages (W. Creyaufmüller)
• Lehmer Five (W. Creyaufmüller)
• Tables of Aliquot Cycles (J.O.M. Pedersen)
• Catalan's Aliquot Sequence Conjecture from MathWorld (E.W. Weisstein)

• 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.