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 A102264 #18 Dec 28 2024 10:20:02
%S A102264 5,17,41,257,521,4481,9521,21929,72089,531977,1256009,5014169,
%T A102264 20879129,70993529,258844361,866941841,3771185921,13949589209,
%U A102264 57388297721
%N A102264 Smallest prime for which 2^n exactly divides the class number h(-4p).
%H A102264 H. Cohn and J. C. Lagarias, <a href="http://dx.doi.org/10.1090/S0025-5718-1983-0717716-8">On the existence of fields governing the 2-invariants of the classgroup of Q(sqrt{dp}) as p varies</a>, Math. Comp. 41 (1983), 711-730.
%H A102264 S. Louboutin, <a href="http://dx.doi.org/10.1090/S0025-5718-1992-1134735-6">L-functions and class numbers of imaginary quadratic fields and of quadratic extensions of an imaginary quadratic field</a>, Math. Comp. 59 (1992) 213-230, Table 1.
%H A102264 A. S. Mosunov and M. J. Jacobson Jr., <a href="https://doi.org/10.1090/mcom3050">Unconditional class group tabulation of imaginary quadratic fields to |Δ| < 2^40</a>, Math. Comp. 85 (2016), no. 300, 1983-2009.
%o A102264 (Sage)
%o A102264 def a(n):
%o A102264 for p in Primes():
%o A102264 if QuadraticField(-p).class_number().valuation(2)==n:
%o A102264 return p # _Robin Visser_, May 25 2024
%Y A102264 Cf. A006641.
%K A102264 nonn,more
%O A102264 1,1
%A A102264 _N. J. A. Sloane_, Feb 19 2005
%E A102264 a(10)-a(17) from _Robin Visser_, May 25 2024
%E A102264 a(18)-a(19) from _Robin Visser_, Dec 28 2024