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Friendly numbers that are smallest in their clubs, ordered by the second-smallest member of the clubs.
1

%I #4 Jul 14 2015 16:39:17

%S 6,30,80,40,12,84,66,78,102,114,240,138,120,150,174,135,186,864,222,

%T 246,60,258,282,560,318,330,354,366,720,390,402,426,360,438,880,474,

%U 498,510,440,534,132,1040,570,582,606,618,520,924,642,654,678,690,540,726

%N Friendly numbers that are smallest in their clubs, ordered by the second-smallest member of the clubs.

%C Subsequence of A259917 (see that entry).

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Friendly_number">Friendly number</a>.

%o (PARI) known=List();for(i=1,10^5,a=sigma(i)/i;match=0;for(j=1,#known,if(known[j][1]==a,match=j;break()));if(match,old=known[match][2];if(old,print1(old,", ");known[match]=[a,0]),listput(known,[a,i])))

%Y Cf. A259917, A050973, A095301, A014567.

%K nonn

%O 1,1

%A _Jeppe Stig Nielsen_, Jul 08 2015