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 A206367 #19 Sep 12 2023 04:39:32 %S A206367 3,15,18,36,72,255,1152,2709,65535,294912,4294967295,4380480000 %N A206367 (2,3)-imperfect numbers. %C A206367 a(13) > 10^10, if it exists. - _Amiram Eldar_, Sep 12 2023 %H A206367 László Tóth, <a href="http://arxiv.org/abs/1111.4842">A survey of the alternating sum-of-divisors function</a>, arXiv:1111.4842 [math.NT], 2011-2014. See p. 7. %t A206367 f[p_, e_] := ((-1)^e + p^(e + 1))/(p + 1); s[n_] := Times @@ f @@@ FactorInteger[n]; s[1] = 1; Select[Range[300000], # == 3 * s[s[#]] &] (* _Amiram Eldar_, Sep 12 2023 *) %o A206367 (PARI) beta(n) = sumdiv(n, d, d*(-1)^bigomega(n/d)); %o A206367 isok(k) = 3*beta(beta(k)) == k; \\ _Michel Marcus_, Sep 11 2023 %Y A206367 Cf. A127725 (2-imperfect). %Y A206367 Cf. A206369 (beta). %K A206367 nonn,more %O A206367 1,1 %A A206367 _N. J. A. Sloane_, Feb 06 2012 %E A206367 a(7)-a(10) from _Michel Marcus_, Sep 11 2023 %E A206367 a(11)-a(12) from _Amiram Eldar_, Sep 12 2023