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 A335251 #11 May 29 2020 11:08:37 %S A335251 1,20,35,143,208,2623,5183,27796,11177983,69677008,920158207, %T A335251 1099508482048 %N A335251 Numbers k such that k and k+1 have the same unitary abundance (A129468). %C A335251 Are there any unitary abundant numbers (A034683) in this sequence? %C A335251 a(12) > 10^11. %C A335251 a(13) > 8*10^12. Also terms: 2^36 * 68719644673, 2^48 * 281474901625261, 2^64 * 18446632096776339457. - _Giovanni Resta_, May 29 2020 %e A335251 1 is a term since 1 and 2 have the same unitary abundance: A129468(1) = usigma(1) - 2*1 = 1 - 2 = -1, and A129468(2) = usigma(2) - 2*2 = 3 - 4 = -1. %t A335251 usigma[1] = 1; usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); udef[n_] := 2*n - usigma[n]; Select[Range[30000], udef[#] == udef[# + 1] &] %Y A335251 Cf. A034448, A034683, A129468, A129487, A331412, A335252. %K A335251 nonn,more %O A335251 1,2 %A A335251 _Amiram Eldar_, May 28 2020 %E A335251 a(12) from _Giovanni Resta_, May 29 2020