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A050973 Larger member of friendly pairs ordered by smallest maximal element. 15
28, 140, 200, 224, 234, 270, 308, 364, 476, 496, 496, 532, 600, 644, 672, 700, 812, 819, 868, 936, 1036, 1148, 1170, 1204, 1316, 1400, 1484, 1488, 1488, 1540, 1638, 1638, 1638, 1652, 1708, 1800, 1820, 1876, 1988, 2016, 2044, 2200, 2212, 2324 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Perfect numbers greater than 6 (A000396) belong to this sequence as they form friendly pairs with smaller perfect, so that the n-th perfect number will appear n-1 times in the sequence. - Michel Marcus, Dec 03 2013

If we remove duplicates from the sequence we get A095301. - Jeppe Stig Nielsen, Jul 08 2015

It is possible to derive a friendly pair from 2 existing pairs (a_n,b_n) and (a_k,b_k); if (a_n,b_k) and (a_k,b_n) (resp. (a_k,b_k) and (a_n,b_n)) are coprime, then (a_n*b_k,a_k*b_n) (resp. (a_k*b_k,a_n*b_n)) is a friendly pair. For instance one can derive (32760,30240) from (819,135) and (224,40). Moreover, since 32760/35 and 30240/35 are both coprime to 35, one can also derive the primitive friendly pair (936,864). - Michel Marcus, Oct 09 2015

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Friendly Pair.

PROG

(PARI) lista(nn) = {for (n=1, nn, ab = sigma(n)/n; for (i=2, n-1, if (sigma(i)/i == ab, print1(n, ", ")); ); ); } \\ Michel Marcus, Dec 03 2013

CROSSREFS

Cf. A050972, A074902, A095301.

Sequence in context: A074233 A201095 A239205 * A095301 A201102 A042530

Adjacent sequences:  A050970 A050971 A050972 * A050974 A050975 A050976

KEYWORD

nonn

AUTHOR

Eric W. Weisstein

STATUS

approved

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Last modified August 19 23:35 EDT 2017. Contains 290821 sequences.