For Seifert manifold $M=X({ p_1}/_{ \f{ q_1}},{ p_2}/_{ \f{ q_2}}, ...,{ p_n}/_ { \f{ q_n}}), \tau^{ '}_r(M)$ is calculated for all $r$ odd $\geq 3$. If $r$ is coprime to at least $n-2$ of $p_k$ (e.g. when $M$ is the Poincare homology sphere), it is proved that $(\sqrt { \dfrac{ 4}{ r}}\sin \dfrac{ \pi}{ r})^{ \nu}\tau^{ '}_r(M)$ is an algebraic integer in the r-th cyclotomic field, where $\nu$ is the first Betti number of $M$. For the torus bundle obtained from trefoil knot with framing 0, i.e. $X_{ tref}(0)=X(-2/_{ \f{ 1}},3/_{ \f{ 1}},6/_{ \f{ 1}}), \tau^{ '}_r$ is obtained in a simple form if $3\mid\llap /r$, which shows in some sense that it is impossible to generalize Ohtsuki's invariant to 3-manifolds being not rational homology spheres.
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