We study the capitulation of 2-ideal classes of an infinite family of imaginary bicyclic biquadratic number fields consisting of fields k = Q( √ 2pq, i), where i = √ −1 and p ≡ −q ≡ 1 (mod 4) are different primes. For each of the three quadratic extensions K/k inside the absolute genus field k (∗) of k, we determine a fundamental system of units and then compute the capitulation kernel of K/k. The generators of the groups Ams(k/F) and Am(k/F) are also determined from which we deduce that k (∗) is smaller than the relative genus field (k/Q(i))∗ . Then we prove that each strongly ambiguous class of k/Q(i) capitulates already in k (∗) , which gives an example generalizing a theorem of Furuya (1977).
Let $p_1\equiv p_2\equiv 1 \pmod 8$ be primes such that $(\frac {p_1}{p_2})=-1$ and $(\frac {2}{a+b})=-1$, where $p_1p_2=a^2+b^2$. Let ${\rm i}=\sqrt {-1}$, $d=p_1p_2$, $\Bbbk =\mathbb {Q}(\sqrt {d},{\rm i})$, $\Bbbk _2^{(1)}$ be the Hilbert 2-class field and $\Bbbk ^{(*)}=\mathbb{Q} (\sqrt {p_1},\sqrt {p_2},{\rm i})$ be the genus field of $\Bbbk $. The 2-part ${\bf C}_{{\Bbbk },2}$ of the class group of $\Bbbk $ is of type $(2,2,2)$, so $\Bbbk _2^{(1)}$ contains seven unramified quadratic extensions $\mathbb K_j/\Bbbk $ and seven unramified biquadratic extensions $\mathbb {L}_j/\Bbbk $. Our goal is to determine the fourteen extensions, the group ${\bf C}_{{\Bbbk },2}$ and to study the capitulation problem of the 2-classes of $\Bbbk $. \medskip {\it Résumé. Soient $p_1\equiv p_2\equiv 1\pmod 8$ des nombres premiers tels que, $(\frac {p_1}{p_2})=-1$ et $(\frac {2}{a+b})=-1$, où $p_1p_2=a^2+b^2$. Soient ${\rm i}=\sqrt {-1}$, $d=p_1p_2$, $\Bbbk =\mathbb {Q}(\sqrt {d},{\rm i})$, $\Bbbk _2^{(1)}$ le 2-corps de classes de Hilbert de $\Bbbk $ et $\Bbbk ^{(*)}=\mathbb{Q} (\sqrt {p_1},\sqrt {p_2},{\rm i})$ le corps de genres de $\Bbbk $. La 2-partie ${\bf C}_{{\Bbbk },2}$ du groupe de classes de $\Bbbk $ est de type $(2, 2, 2)$, par suite $\Bbbk _2^{(1)}$ contient sept extensions quadratiques non ramifiées $\mathbb K_j/\Bbbk $ et sept extensions biquadratiques non ramifiées $\mathbb {L}_j/\Bbbk $. Dans ce papier on s'intéresse à déterminer ces quatorze extensions, le groupe ${\bf C}_{{\Bbbk },2}$ et à étudier la capitulation des 2-classes d'idéaux de $\Bbbk $ dans ces extensions. and Soient $p_1\equiv p_2\equiv1\pmod8$ des nombres premiers tels que, $(\frac{p_1}{p_2})=-1$ et $(\frac2{a+b})=-1$, où $p_1p_2=a^2+b^2$. Soient $ i=\sqrt{-1}$, $d=p_1p_2$, $\Bbbk=\mathbb{Q}(\sqrt{d}, i)$, $\Bbbk_2^{(1)}$ le 2-corps de classes de Hilbert de $\Bbbk$ et $\Bbbk^{(*)}=\mathbb Q(\sqrt{p_1},\sqrt{p_2}, i)$ le corps de genres de $\Bbbk$. La 2-partie $ C_{{\Bbbk},2}$ du groupe de classes de $\Bbbk$ est de type $(2, 2, 2)$, par suite $\Bbbk_2^{(1)}$ contient sept extensions quadratiques non ramifiées $\mathbb K_j/\Bbbk$ et sept extensions biquadratiques non ramifiées $\mathbb{L}_j/\Bbbk$. Dans ce papier on s'intéresse à déterminer ces quatorze extensions, le groupe $ C_{{\Bbbk},2}$ et à étudier la capitulation des 2-classes d'idéaux de $\Bbbk$ dans ces extensions.