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 A231791 #21 Feb 13 2024 23:16:18 %S A231791 8,25,77,125,133,209,301,325,425,469,473,725,737,817,925,1025,1141, %T A231791 1273,1325,1525,1625,1793,1825,2125,2225,2425,2525,2725,2825,2881, %U A231791 3097,3425,3625,3725,3925,4325,4525,4625,4825,4925,5125,5525,5725,5825,6025,6425 %N A231791 Integers k such that A231589(k) = floor(k*(k-1)/4) - k. %C A231791 It appears that this sequence is the union of 3 sets. %C A231791 First term is 8, and is the only even known value. %C A231791 Then we get terms that are equal to 25 * b with b a squarefree product of primes congruent to 1 modulo 4 (A002144), that is, terms of A231754. %C A231791 And we get the following terms 77, 133, 209, 301, 469, 473, 737, 817, 1141, 1273, 1793, 2881, 3097, 7009, 10921. These numbers are the products of 2 distinct primes from this list: 7, 11, 19, 43, 67, 163 (a subsequence of A003173). %H A231791 Hugo Pfoertner, <a href="/A231791/b231791.txt">Table of n, a(n) for n = 1..10000</a> %H A231791 John P. Robertson, <a href="https://web.archive.org/web/20180831181644/http://www.jpr2718.org/shir2.pdf">Shirali’s Questions About Sums of Residues of Squares</a> %H A231791 S. A. Shirali, <a href="http://www.jstor.org/stable/2690862">A family portrait of primes-a case study in discrimination</a>, Math. Mag. Vol. 70, No. 4 (Oct., 1997), pp. 263-272. %o A231791 (PARI) isok(n) = A231589(n) == n*(n-1)/4 - n; %Y A231791 Cf. A231589. %K A231791 nonn %O A231791 1,1 %A A231791 _Michel Marcus_, Nov 13 2013