

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
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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 nth perfect number will appear n1 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, n1, 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 A317680
Adjacent sequences: A050970 A050971 A050972 * A050974 A050975 A050976


KEYWORD

nonn


AUTHOR

Eric W. Weisstein


STATUS

approved



