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A279812 Let s(k) denote the sum of the even proper divisors of k. The sequence lists the pairs of numbers (x, y) such that s(x) = y and s(y) = x. 1
440, 568, 2368, 2420, 5240, 5848, 10040, 11128, 12464, 12736, 21488, 21712, 24570, 29190, 34592, 36832, 126040, 133856, 133984, 134190, 139230, 142290, 152168, 159500, 175266, 177460, 200970, 244530, 244736, 246304, 248310, 279630, 283328, 284620, 306352, 337460 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

"Pseudo-amicable" numbers: pairs of numbers (x, y) such that A091818(x) = y and A091818(y) = x.

A pair of numbers x and y is called "pseudo-amicable" if the sum of the proper even divisors of either one is equal to the other. The smallest pair is x = 440, y = 568.

The sequence lists the numbers in increasing order. Note that the pairs (x, y) are not adjacent to each other in the list. For instance, the pair (126040, 152168) are "pseudo-amicable" numbers.

For ordered pairs see A279950.

LINKS

Table of n, a(n) for n=1..36.

EXAMPLE

The pair (a(1), a(2)) = (440, 568) is in the sequence because the even proper divisors of 440 are 2, 4, 8, 10, 20, 22, 40, 44, 88, 110, 220 with sum = 568 and the proper divisors of 568 are 2, 4, 8, 142, 284 with sum = 440.

MATHEMATICA

s[n_]:=2*(DivisorSigma[1, n/2]-n/2); P[n_]:=If[Nest[s, n, 2]==n&&!s[n]==n, True, False]; Select[Range[10^6], P[#]&]

CROSSREFS

Cf. A063990, A074400, A091818, A279950.

Sequence in context: A210205 A092048 A296905 * A279950 A072604 A158319

Adjacent sequences:  A279809 A279810 A279811 * A279813 A279814 A279815

KEYWORD

nonn

AUTHOR

Michel Lagneau, Dec 19 2016

STATUS

approved

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Last modified September 18 10:31 EDT 2020. Contains 337166 sequences. (Running on oeis4.)