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 A235796 #53 Sep 08 2022 08:46:06
%S A235796 0,0,1,0,3,-1,5,0,4,1,9,-5,11,3,5,0,15,-4,17,-3,9,7,21,-13,18,9,13,-1,
%T A235796 27,-13,29,0,17,13,21,-20,35,15,21,-11,39,-13,41,3,11,19,45,-29,40,6,
%U A235796 29,5,51,-13,37,-9,33,25,57,-49,59,27,21,0,45,-13,65,9,41
%N A235796 2*n - 1 - sigma(n).
%C A235796 Partial sums give A004125.
%C A235796 Also 0 together with A120444.
%C A235796 It appears that a(n) = 0 iff n is a power of 2.
%C A235796 Numbers n with a(n) = 0 are called "almost perfect", "least deficient" or "slightly defective" numbers. See A000079. - _Robert Israel_, Jul 22 2014
%C A235796 a(n) = n - 2 iff n is prime.
%C A235796 a(n) = -1 iff n is a perfect number.
%C A235796 Also the alternating row sums of A239446. - _Omar E. Pol_, Jul 21 2014
%D A235796 R. K. Guy, Unsolved Problems in Number Theory, 3rd Edition, Springer, New York, 2004.
%H A235796 T. D. Noe, <a href="/A235796/b235796.txt">Table of n, a(n) for n = 1..10000</a>
%F A235796 a(n) = A005408(n-1) - A000203(n).
%F A235796 a(n) = -1 - A033880(n). - _Michel Marcus_, Jan 27 2014
%F A235796 a(n) = n - 1 - A001065(n). - _Omar E. Pol_, Jan 29 2014
%F A235796 a(n) = A033879(n) - 1. - _Omar E. Pol_, Jan 30 2014
%F A235796 a(n) = 2*n - 2 - A039653(n). - _Omar E. Pol_, Jan 31 2014
%F A235796 a(n) = (-1)*A237588(n). - _Omar E. Pol_, Feb 23 2014
%F A235796 a(n) = 2*n - A088580(n). - _Omar E. Pol_, Mar 23 2014
%e A235796 . The positive The sum of
%e A235796 n odd numbers divisors of n. a(n)
%e A235796 1 1 1 0
%e A235796 2 3 3 0
%e A235796 3 5 4 1
%e A235796 4 7 7 0
%e A235796 5 9 6 3
%e A235796 6 11 12 -1
%e A235796 7 13 8 5
%e A235796 8 15 15 0
%e A235796 9 17 13 4
%e A235796 10 19 18 1
%e A235796 ...
%t A235796 Table[2n-1-DivisorSigma[1,n],{n,70}] (* _Harvey P. Dale_, Jul 11 2014 *)
%o A235796 (PARI) vector(100, n, (2*n-1)-sigma(n)) \\ _Colin Barker_, Jan 27 2014
%o A235796 (Magma) [2*n-1-SumOfDivisors(n): n in [1..100]]; // _Vincenzo Librandi_, Feb 25 2014
%Y A235796 Cf. A000079, A000203, A000396, A004125, A005408, A120444, A196020, A236104.
%K A235796 sign
%O A235796 1,5
%A A235796 _Omar E. Pol_, Jan 25 2014