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 A381548 #11 Apr 26 2025 05:24:52 %S A381548 5984,7424,21735,27404,43064,56924,76544,82004,89775,109395,144584, %T A381548 158235,164835,165375,174824,222704,266475,271215,300104,311024, %U A381548 322335,326655,326864,334304,347984,350175,371924,387584,393855,414315,442035,445004,447524,477224 %N A381548 Numbers k such that k and k+1 both have an odd number of abundant divisors. %C A381548 Numbers k such that k and k+1 are both in A381546. %H A381548 Amiram Eldar, <a href="/A381548/b381548.txt">Table of n, a(n) for n = 1..10000</a> %e A381548 5984 is a term since it has 9 abundant divisors (88, 176, 272, 352, 544, 748, 1496, 2992, 5984) and 5984 + 1 = 5985 has one abundant divisor (5985 itself). %e A381548 21735 is a term since it has 3 abundant divisors (945, 7245, 21735) and 21735 + 1 = 21736 has 9 abundant divisors (88, 104, 572, 836, 1144, 1672, 1976, 10868, 21736). %t A381548 q[n_] := q[n] = OddQ[DivisorSum[n, 1 &, DivisorSigma[-1, #] > 2 &]]; Select[Range[500000], q[#] && q[#+1] &] %o A381548 (PARI) is1(k) = sumdiv(k, d, (sigma(d, -1) > 2)) % 2; %o A381548 list(lim) = forstep(k = 3, lim, 2, if(is1(k), if(is1(k-1), print1(k-1, ", ")); if(is1(k+1), print1(k, ", ")))); %Y A381548 Subsequence of A096399 and A381546. %Y A381548 Subsequences: A381549. %K A381548 nonn %O A381548 1,1 %A A381548 _Amiram Eldar_, Feb 26 2025