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 A381546 #8 Feb 27 2025 06:41:24 %S A381546 12,18,20,30,36,42,48,56,66,70,72,78,80,84,88,90,102,104,108,114,120, %T A381546 126,132,138,140,144,156,162,174,186,192,196,198,204,222,224,228,234, %U A381546 246,252,258,270,272,276,282,288,300,304,306,308,318,320,324,330,336,342,348 %N A381546 Numbers with an odd number of abundant divisors. %C A381546 Numbers k such that A080224(k) is odd. %C A381546 The primitive abundant numbers (A091191) are all terms of this sequence since A080224(A091191(n)) = 1 for all n. %H A381546 Amiram Eldar, <a href="/A381546/b381546.txt">Table of n, a(n) for n = 1..10000</a> %e A381546 12 is a term since it has only one abundant divisor, 12 itself. %e A381546 36 is a term since it has 3 abundant divisors, 12, 18 and 36. %e A381546 72 is a term since it has 5 abundant divisors, 12, 18, 24, 36 and 72. %t A381546 q[n_] := OddQ[DivisorSum[n, 1 &, DivisorSigma[-1, #] > 2 &]]; Select[Range[350], q] %o A381546 (PARI) isok(k) = sumdiv(k, d, (sigma(d, -1) > 2)) % 2; %Y A381546 Cf. A080224. %Y A381546 Subsequence of A005101. %Y A381546 Subsequences: A091191, A381547, A381548, A381549. %K A381546 nonn,easy %O A381546 1,1 %A A381546 _Amiram Eldar_, Feb 26 2025