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With K=Q((3812377)^(1/2)) we give the first example of an algebraic numberfield possessing a 5-class tower of exact length L(5,K)=3. The rigorous proofis conducted by means of the p-group generation algorithm, showing theexistence of a unique finite metabelian 5-group G with abelianization [5,5]having the kernels (M(1),G^5) and targets ([25,5,5,5],[5,5]^5) of Artintransfers T(i):G-->M(i)/M(i)' to its six maximal subgroups M(i), prescribed byarithmetical invariants of K. Thus, G must be the second 5-class group G(5,2,K)of the real quadratic field K but cannot be its 5-class tower group G(5,K),since the relation rank d(2,G)=4 is too big. We provide evidence of exactlyfive non-isomorphic extensions H of G having the required relation rankd(2,H)=3 and derived length dl(H)=3 whose metabelianization H/H'' is isomorphicto G. Consequently, G(5,K) must be one of the five non-metabelian groups H.

Three stage towers of 5-class fields