%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