This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A064012 #17 Apr 10 2019 02:45:58 %S A064012 10,30,288,660,720,2146560 %N A064012 usigma(usigma(n))=3*n where usigma(n) is the sum of unitary divisors of n (A034448). %C A064012 Is this sequence finite? %C A064012 18 is the only known integer usigma(usigma(n)) = kn for some integer k >= 4 (no other one <= 2^30). - _Tomohiro Yamada_, Apr 22 2017 %C A064012 a(7) > 2.85*10^11, if it exists. The same bound holds also for any n > 18 such usigma(usigma(n)) = k*n for some integer k >= 4. - _Giovanni Resta_, Apr 10 2019 %H A064012 V. Sitaramaiah and M. V. Subbarao, <a href="http://www.math.ualberta.ca/~subbarao/documents/Sitaramaiah_Subbarao1998.pdf">On the equation usigma(usigma(n)) = 2n</a>, Util. Math. 53 (1998), 101--124. %e A064012 10 belongs to the sequence because usigma(usigma(10)) = 30 = 3*10. %Y A064012 Cf. A034448, A038843. %K A064012 nonn %O A064012 1,1 %A A064012 _Felice Russo_, Sep 07 2001 %E A064012 One more term from _Naohiro Nomoto_, Oct 21 2001 %E A064012 No other terms < 1070000000, _Jud McCranie_, Oct 28 2001