This site is supported by donations to The OEIS Foundation.



Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 4500 articles have referenced us, often saying "we would not have discovered this result without the OEIS".

(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A063990 Amicable numbers. 51
220, 284, 1184, 1210, 2620, 2924, 5020, 5564, 6232, 6368, 10744, 10856, 12285, 14595, 17296, 18416, 63020, 66928, 66992, 67095, 69615, 71145, 76084, 79750, 87633, 88730, 100485, 122265, 122368, 123152, 124155, 139815, 141664, 142310 (list; graph; refs; listen; history; text; internal format)



A pair of numbers x and y is called amicable if the sum of the proper divisors of either one is equal to the other. The smallest pair is x = 220, y = 284.

The sequence lists the amicable numbers in increasing order. Note that the pairs x, y are not adjacent to each other in the list. See also A002025 for the x's, A002046 for the y's.

Theorem: If the three numbers p = 3*(2^(n-1)) - 1, q = 3*(2^n) - 1 and r = 9*(2^(2n-1)) - 1 are all prime where n >= 2, then p*q*(2^n) and r*(2^n) are amicable numbers. This 9th century theorem is due to Thabit ibn Kurrah (see for example, the History of Mathematics by David M. Burton, 6th ed., p. 510). - Mohammad K. Azarian, May 19 2008

The first time a pair ordered by its first element is not adjacent is x = 63020, y = 76084 which correspond to a(17) and a(23), respectively. - Omar E. Pol, Jun 22 2015

For amicable pairs see A259180 and also A259933. - Omar E. Pol, Jul 15 2015


Scott T. Cohen, Mathematical Buds, Ed. H. D. Ruderman, Vol. 1 Chap. VIII pp. 103-126 Mu Alpha Theta 1984.

Clifford A. Pickover, The Math Book, Sterling, NY, 2009; see p. 90.

David Wells, The Penguin Dictionary of Curious and Interesting Numbers, pp. 145-7, Penguin Books 1987.


T. D. Noe, Table of n, a(n) for n = 1..77977 (terms < 10^14 from Pedersen's tables)

Titu Andreescu, Number Theory Trivia: Amicable Numbers

Titu Andreescu, Number Theory Trivia: Amicable Numbers

Anonymous, Amicable Pairs Applet Test

Anonymous, Amicable and Social Numbers

Sergei Chernykh, Table of n, a(n) for n = 1..823818, zipped file (results of an exhaustive search for all amicable pairs with smaller member < 10^17)

Sergei Chernykh, Amicable pairs list

Germano D'Abramo, On Amicable Numbers With Different Parity, arXiv:math/0501402 [math.HO], 2005-2007.

Leonhard Euler, On amicable numbers, arXiv:math/0409196 [math.HO], 2004-2009.

Steven Finch, Amicable Pairs and Aliquot Sequences, 2013.

Mariano García, A Million New Amicable Pairs, J. Integer Sequences, 4 (2001), #01.2.6.

Mariano García, Jan Munch Pedersen, Herman te Riele, Amicable pairs, a survey, Report MAS-R0307, Centrum Wiskunde & Informatica.

Hisanori Mishima, Amicable Numbers:first 236 pairs(smaller member<10^8) fully factorized

David Moews, A List Of The First 5001 Amicable Pairs

David and P. C. Moews, A List Of Amicable Pairs Below 2.01*10^11

Number Theory List, NMBRTHRY Archives--August 1993

Jan Munch Pedersen, Known Amicable Pairs [Broken link]

Jan Munch Pedersen, Tables of Aliquot Cycles [Broken link]

Ivars Peterson, MathTrek, Appealing Numbers

Ivars Peterson, MathTrek, Amicable Pairs, Divisors and a New Record

Carl Pomerance, On amicable numbers (2015)

Herman J. J. te Riele, On generating new amicable pairs from given amicable pairs, Math. Comp. 42 (1984), 219-223.

Herman J. J. te Riele, Computation of all the amicable pairs below 10^10, Math. Comp., 47 (1986), 361-368 and Supplement pp. S9-S40.

Herman J. J. te Riele, A New Method for Finding Amicable Pairs, Proceedings of Symposia in Applied Mathematics, Volume 48, 1994.

Ed Sandifer, Amicable numbers

Gérard Villemin's Almanach of Numbers, Nombres amiables et sociables

Eric Weisstein's World of Mathematics, Amicable Pair

Wikipedia, Amicable number


Pomerance shows that there are at most x/exp(sqrt(log x log log log x)/(2 + o(1))) terms up to x for sufficiently large x. - Charles R Greathouse IV, Jul 21 2015


F:= proc(t) option remember; numtheory:-sigma(t)-t end proc:

select(t -> F(t) <> t and F(F(t))=t, [$1.. 200000]); # Robert Israel, Jun 22 2015


s[n_] := DivisorSigma[1, n] - n; AmicableNumberQ[n_] := If[Nest[s, n, 2] == n && ! s[n] == n, True, False]; Select[Range[10^6], AmicableNumberQ[ # ] &] (* Ant King, Jan 02 2007 *)


(PARI) aliquot(n)=sigma(n)-n

isA063990(n)={local(a); a=aliquot(n); a<>n && aliquot(a)==n} \\ Michael B. Porter, Apr 13 2010


from sympy import divisors

A063990 = [n for n in xrange(1, 10**5) if sum(divisors(n))-2*n and not sum(divisors(sum(divisors(n))-n))-sum(divisors(n))] # Chai Wah Wu, Aug 14 2014


Union of A002025 and A002046.

A180164 (gives for each pair (x, y) the value x+y = sigma(x)+sigma(y)).

Sequence in context: A203777 A121507 A255215 * A259180 A259933 A262624

Adjacent sequences:  A063987 A063988 A063989 * A063991 A063992 A063993




N. J. A. Sloane, Sep 18 2001


Comment about the first not adjacent pair being (67095, 71145) removed by Michel Marcus, Aug 21 2015



Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified November 25 08:56 EST 2015. Contains 264417 sequences.