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 A035203 #19 Nov 19 2023 01:29:05
%S A035203 1,0,1,1,2,0,1,0,1,0,0,1,0,0,2,1,2,0,0,2,1,0,0,0,3,0,1,1,0,0,0,0,0,0,
%T A035203 2,1,2,0,0,0,2,0,2,0,2,0,2,1,1,0,2,0,0,0,0,0,0,0,2,2,0,0,1,1,0,0,2,2,
%U A035203 0,0,0,0,0,0,3,0,0,0,2,2,1
%N A035203 Coefficients in expansion of Dirichlet series Product_p (1-(Kronecker(m,p)+1)*p^(-s)+Kronecker(m,p)*p^(-2s))^(-1) for m = 21.
%C A035203 Coefficients of Dedekind zeta function for the quadratic number field of discriminant 21. See A002324 for formula and Maple code. - _N. J. A. Sloane_, Mar 22 2022
%H A035203 Amiram Eldar, <a href="/A035203/b035203.txt">Table of n, a(n) for n = 1..10000</a>
%F A035203 Asymptotic mean: Limit_{m->oo} (1/m) * Sum_{k=1..m} a(k) = 2*log((5+sqrt(21))/2)/sqrt(21) = 0.683807... . - _Amiram Eldar_, Oct 11 2022
%F A035203 From _Amiram Eldar_, Nov 19 2023: (Start)
%F A035203 a(n) = Sum_{d|n} Kronecker(21, d).
%F A035203 Multiplicative with a(p^e) = 1 if Kronecker(21, p) = 0 (p = 3 or 7), a(p^e) = (1+(-1)^e)/2 if Kronecker(21, p) = -1 (p is in A038894), and a(p^e) = e+1 if Kronecker(21, p) = 1 (p is in A038893 \ {3, 7}). (End)
%t A035203 a[n_] := DivisorSum[n, KroneckerSymbol[21, #] &]; Array[a, 100] (* _Amiram Eldar_, Oct 11 2022 *)
%o A035203 (PARI) my(m = 21); direuler(p=2,101,1/(1-(kronecker(m,p)*(X-X^2))-X))
%o A035203 (PARI) a(n) = sumdiv(n, d, kronecker(21, d)); \\ _Amiram Eldar_, Nov 19 2023
%Y A035203 Dedekind zeta functions for imaginary quadratic number fields of discriminants -3, -4, -7, -8, -11, -15, -19, -20 are A002324, A002654, A035182, A002325, A035179, A035175, A035171, A035170, respectively.
%Y A035203 Dedekind zeta functions for real quadratic number fields of discriminants 5, 8, 12, 13, 17, 21, 24, 28, 29, 33, 37, 40 are A035187, A035185, A035194, A035195, A035199, A035203, A035188, A035210, A035211, A035215, A035219, A035192, respectively.
%Y A035203 Cf. A038893, A038894.
%K A035203 nonn,easy,mult
%O A035203 1,5
%A A035203 _N. J. A. Sloane_