Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #12 May 25 2024 21:05:46
%S 5,17,41,257,521,4481,9521,21929,72089,531977,1256009,5014169,
%T 20879129,70993529,258844361,866941841,3771185921
%N Smallest prime for which 2^n exactly divides the class number h(-4p).
%H 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 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
%o (Sage)
%o def a(n):
%o for p in Primes():
%o if QuadraticField(-p).class_number().valuation(2)==n:
%o return p # _Robin Visser_, May 25 2024
%Y Cf. A006641.
%K nonn,more
%O 1,1
%A _N. J. A. Sloane_, Feb 19 2005
%E a(10)-a(17) from _Robin Visser_, May 25 2024