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 A218381 #21 Nov 06 2024 04:39:27 %S A218381 3,4,8,14,15,18,20,24,28,35,39,46,48,55,60,63,64,66,68,80,84,94,99, %T A218381 100,114,120,124,126,128,136,138,143,144,150,154,155,156,158,168,180, %U A218381 183,195,196,203,220,224,234,238,240,243,255,258,260,275,284,288,291 %N A218381 Numbers k such that A211996(k) is not zero. %C A218381 For any n, the equation x^4 + a(n)*y^4 = z^2 is solvable in integers. - _Arkadiusz Wesolowski_, Aug 15 2013 %C A218381 The asymptotic density of this sequence is 0 (De Koninck et al., 2024). - _Amiram Eldar_, Nov 05 2024 %H A218381 Amiram Eldar, <a href="/A218381/b218381.txt">Table of n, a(n) for n = 1..10000</a> %H A218381 David Clark, <a href="http://dx.doi.org/10.4153/CMB-1991-029-4">An arithmetical function associated with the rank of elliptic curves</a>, Canad. Math. Bull. Vol. 34 (2), (1991), pp. 181-185. %H A218381 Jean-Marie De Koninck, A. Arthur Bonkli Razafindrasoanaivolala, and Hans Schmidt Ramiliarimanana, <a href="https://doi.org/10.1007/s40993-024-00520-x">Integers with a sum of co-divisors yielding a square</a>, Research in Number Theory, Vol. 10, No. 2 (2024), Article 30; <a href="https://www.jeanmariedekoninck.mat.ulaval.ca/fileadmin/Documents/Publications/2023_integers_with_a_sum_of_co-divisors_yielding_a_square.pdf">author's copy</a>. %t A218381 q[k_] := DivisorSum[k, 1 &, #^2 <= k && IntegerQ[Sqrt[# + k/#]] &] > 0; Select[Range[300], q] (* _Amiram Eldar_, Nov 05 2024 *) %o A218381 (PARI) is(k) = k > 1 && fordiv(k, d, if(issquare(d + k/d), return(1)); if(d^2 > k, return(0))); \\ _Amiram Eldar_, Nov 05 2024 %Y A218381 Cf. A211996, A218382, A228880. %K A218381 nonn %O A218381 1,1 %A A218381 _Michel Marcus_, Oct 27 2012