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 A003642 M0211 #18 Jul 25 2019 12:26:13 %S A003642 1,1,2,2,2,2,2,2,4,2,2,2,4,4,2,2,2,2,4,2,2,4,2,2,2,4,4,4,4,2,2,4,4,2, %T A003642 4,2,2,4,2,2,2,4,8,2,2,4,2,4,2,2,4,4,4,2,2,4,4,2,4,2,2,4,2,2,4,8,2,4, %U A003642 2,4,4,2,2,4,4,4,4,2,2,2,4,2,4,2,4,8,4,2,4,2,4,4,2,2,4,2,4,4,2,4,4,4,2,2,4 %N A003642 Number of genera of imaginary quadratic field with discriminant -k, k = A191483(n). %D A003642 D. A. Buell, Binary Quadratic Forms. Springer-Verlag, NY, 1989, pp. 224-241. %D A003642 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A003642 <a href="/index/Qua#quadfield">Index entries for sequences related to quadratic fields</a> %F A003642 a(n) = 2^(omega(A191483(n)) - 1). - _Jianing Song_, Jul 24 2018 %t A003642 2^(PrimeNu[Select[Range[1000], Mod[#, 4] == 0 && SquareFreeQ[#/4] && Mod[#, 16] != 12&]] - 1) (* _Jean-François Alcover_, Jul 25 2019, after _Andrew Howroyd_ in A191483 *) %o A003642 (PARI) for(n=1, 1000, if(isfundamental(-n) && n%2==0, print1(2^(omega(n) - 1), ", "))) \\ _Andrew Howroyd_, Jul 24 2018 %Y A003642 Cf. A001221 (omega), A003640, A003641, A191483. %K A003642 nonn %O A003642 1,3 %A A003642 _N. J. A. Sloane_, _Mira Bernstein_ %E A003642 Name clarified by _Jianing Song_, Jul 24 2018