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 A343239 #19 Jan 06 2024 14:32:10
%S A343239 0,1,1,2,0,1,5,3,4,2,7,5,3,11,5,10,7,11,4,10,11,17,8,15,7,20,13,16,5,
%T A343239 25,10,25,6,35,11,17,25,31,9,34,20,25,15,31,18,29,17,32,7,47,13,45,19,
%U A343239 42,11,25,38,52,14,53,8,31,38,61,25,45,20,61,35,47,24,59,9,77,13,16,71,74
%N A343239 Irregular triangle read by rows giving the solutions x for x^2 == -5 (mod A343238(n)), for x from {0, 1, 2, ..., A343238(n)-1}, for n >= 1.
%C A343239 The length of row n is A343240(n).
%H A343239 Andrew Howroyd, <a href="/A343239/b343239.txt">Table of n, a(n) for n = 1..1468</a> (first 500 rows)
%F A343239 T(n, k) gives the solutions x from {0, 1, ..., A343238(n)-1} of the congruence x^2 + 5 == 0 (mod A343238(n)), for n >= 1.
%e A343239 The irregular triangle T with A(n) = A343238(n) begins:
%e A343239 n A(n) \ k 1 2 3 4 ...
%e A343239 ---------------------------
%e A343239 1, 1: 0
%e A343239 2, 2: 1
%e A343239 3, 3: 1 2
%e A343239 4, 5: 0
%e A343239 5, 6: 1 5
%e A343239 6, 7: 3 4
%e A343239 7, 9: 2 7
%e A343239 8, 10: 5
%e A343239 9, 14: 3 11
%e A343239 10, 15: 5 10
%e A343239 11, 18: 7 11
%e A343239 12, 21: 4 10 11 17
%e A343239 13, 23: 8 15
%e A343239 14, 27: 7 20
%e A343239 15, 29: 13 16
%e A343239 16, 30: 5 25
%e A343239 17, 35: 10 25
%e A343239 18, 41: 6 35
%e A343239 19, 42: 11 17 25 31
%e A343239 20, 43: 9 34
%e A343239 ...
%o A343239 (PARI) isok(k) = issquare(Mod(-5, k)); \\ A343238
%o A343239 lista(nn) = my(list = List()); for (n=1, nn, if (issquare(Mod(-5, n)), my(row = select(x->(Mod(x,n)^2 + 5 == 0), [0..n-1])); listput(list, row))); Vec(list); \\ _Michel Marcus_, Sep 17 2023
%Y A343239 Cf. A343238, A343240.
%K A343239 nonn,tabf,easy
%O A343239 1,4
%A A343239 _Wolfdieter Lang_, May 16 2021