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 A003643 M0194 #25 Sep 04 2019 08:06:42 %S A003643 1,1,1,2,2,1,2,1,2,2,2,2,1,4,2,1,2,2,4,1,4,2,2,2,2,2,2,4,1,2,1,2,2,2, %T A003643 4,2,1,2,2,4,4,1,4,4,1,2,2,4,4,1,2,1,4,2,2,2,2,4,2,2,2,2,4,1,8,2,1,2, %U A003643 4,2,2,4,2,2,2,2,2,1,4,4,1,4,2,2,4,1,4,2,2,4,2,2,1,4,2,2,2,2,4,1,8,2,1,4,2 %N A003643 Number of genera of Q(sqrt(-n)), n squarefree. %D A003643 D. A. Buell, Binary Quadratic Forms. Springer-Verlag, NY, 1989, pp. 224-241. %D A003643 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A003643 Andrew Howroyd, <a href="/A003643/b003643.txt">Table of n, a(n) for n = 1..1000</a> %H A003643 <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a> %F A003643 a(n) = 2^(omega(A033197(n)) - 1). - _Andrew Howroyd_, Jul 24 2018 %F A003643 Let k = A005117(n) be the n-th squarefree number, then a(n) = 2^omega(k) if k == 1 (mod 4) and 2^(omega(k) - 1) otherwise. - _Jianing Song_, Jul 25 2018 %t A003643 Function[If[Mod[#, 4] == 1, 2^PrimeOmega[#], 2^(PrimeOmega[#] - 1)]] /@ Select[Range[200], SquareFreeQ] (* _Jean-François Alcover_, Sep 04 2019 *) %o A003643 (PARI) for(n=1, 200, if(issquarefree(n), print1(2^(omega(n*if((-n)%4>1, 4, 1)) - 1), ", "))) \\ _Andrew Howroyd_, Jul 24 2018 %Y A003643 Cf. A000924, A001221 (omega), A005117, A033197, A003640. %K A003643 nonn,nice %O A003643 1,4 %A A003643 _N. J. A. Sloane_, _Mira Bernstein_