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 A343240 #12 Sep 17 2023 06:10:57
%S A343240 1,1,2,1,2,2,2,1,2,2,2,4,2,2,2,2,2,2,4,2,2,2,2,2,2,2,2,4,2,4,2,2,2,2,
%T A343240 2,4,2,2,2,2,2,2,4,2,2,2,2,4,4,2,4,2,2,4,4,2,4,2,4,2,2,2,2,4,2,2,4,4,
%U A343240 4,2,4,2,2,4
%N A343240 The number of solutions x from {0, 1, ..., A343238(n)-1} of the congruence x^2 + 5 == 0 (mod A343238(n)) is given by a(n).
%C A343240 Row length of irregular triangle A343239.
%F A343240 a(n) = row length of A343239(n), for n >= 1.
%F A343240 a(1) = a(2) = a(4) = a(8) = 1, and otherwise a(n) = 2^{number of distinct primes from A139513}, that is, primes congruent to {1, 3, 7, 9} (mod 20), appearing in the prime factorization of A343238(n).
%e A343240 a(19) = 4 because A343238(19) = 42 = 2*3*7 has 2^(1+1) = 4 solutions from the primes 3 and 7.
%o A343240 (PARI) isok(k) = issquare(Mod(-5, k)); \\ A343238
%o A343240 lista(nn) = my(list = List()); for (n=1, nn, if (issquare(Mod(-5, n)), listput(list, sum(i=0, n-1, Mod(i,n)^2 + 5 == 0)););); Vec(list); \\ _Michel Marcus_, Sep 17 2023
%Y A343240 Cf. A343238, A343239.
%K A343240 nonn,easy
%O A343240 1,3
%A A343240 _Wolfdieter Lang_, May 16 2021