cp's OEIS Frontend

A383366 Smallest of a sociable triple i < j < k such that j = s(i), k = s(j), and i = s(k), where s(k) = A380845(k) - k is the sum of aliquot divisors of k that have the same binary weight as k.

%I A383366 #8 Apr 24 2025 13:20:53
%S A383366 4400700,12963816,29878920,38353800,44973480,51894304,52208520,
%T A383366 67849656,73134432,81685080,100711656,103759848,105096096,113044896,
%U A383366 113161320,114608032,128639034,135465912,135559080,136786200,139242740,148758120,156686088,159628350,171090416
%N A383366 Smallest of a sociable triple i < j < k such that j = s(i), k = s(j), and i = s(k), where s(k) = A380845(k) - k is the sum of aliquot divisors of k that have the same binary weight as k.
%H A383366 Amiram Eldar, <a href="/A383366/b383366.txt">Table of n, a(n) for n = 1..4400</a>
%H A383366 Amiram Eldar, <a href="/A383366/a383366.txt">Table of n, a(n), s(a(n)), s(s(a(n))) for n = 1..4400</a>.
%e A383366 4400700 is a term since s(4400700) = 4840770, s(4840770) = 5456868, and s(5456868) = 4400700.
%t A383366 f[n_] := Module[{h = DigitCount[n, 2, 1]}, DivisorSum[n, # &, # < n && DigitCount[#, 2, 1] == h &]]; q[k_] := Module[{k1 = f[k], k2}, If[k1 <= k, False, k2 = f[k1]; k2 > k && f[k2] == k]]; Select[Range[13000000], q]
%o A383366 (PARI) f(n) = {my(h = hammingweight(n)); sumdiv(n, d, d * (d < n && hammingweight(d) == h)); }
%o A383366 isok(k) = {my(k1 = f(k), k2); if(k1 <= k, 0, k2 = f(k1); k2 > k && f(k2) == k);}
%Y A383366 Cf. A380845, A380846, A380849, A380850.
%K A383366 nonn,base
%O A383366 1,1
%A A383366 _Amiram Eldar_, Apr 24 2025