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Susumu Oda

Publications and source records attributed to Susumu Oda.

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Some Comments around The Examples against The Generalized Jacobian Conjecture

We have studied a faded problem, the Jacobian Conjecture ~: \noindent {\sf The Jacobian Conjecture $(JC_n)$}~: If $f_1, \cdots, f_n$ are elements in a polynomial ring $k[X_1, \cdots, X_n]$ over a field $k$ of characteristic $0$ such that the Jacobian $\det(\partial f_i/ \partial X_j) $ is a nonzero constant, then $k[f_1, \cdots, f_n] = k[X_1, \cdots, X_n]$. For this purpose, we generalize it to the following form~: \noindent {\sf The Generalized Jacobian Conjecture $(GJC)$}~: {\it Let $φ: S \rightarrow T$ be an unramified homomorphism of Noetherian domains with $T^\times = φ(S^\times)$. Assume that $T$ is a factorial domain and that $S$ is a simply connected normal domain. Then $φ$ is an isomorphism. } For the consistency of our discussion, we raise some serious (or idiot) questions and some comments concerning the examples appeared in the papers published by the certain excellent mathematicians (though we are unwilling to deal with them). Since the existence of such examples would be against our original target Conjecture$(GJC)$, we have to dispute their arguments about the existence of their respective (so called) counter-examples. Our conclusion is that they are not perfect counter-examples as are shown explicitly.

math.AC

An Analytic Approach to The Cancellation Problem for Affine Spaces over $\mathbb{C}$

The Cancellation Problem for Affine Spaces is settled affirmatively, that is, it is proved that : Let $ k $ be an algebraically closed field of characteristic zero and let $n, m \in \mathbb{N}$. If $R[Y_1,..., Y_m] \cong_k k[X_1,..., X_{n+m}]$ as $k$-algebras, where $Y_1,..., Y_m, X_1,..., X_{n+m}$ are indeterminates, then $R \cong_k k[X_1,..., X_n]$.

math.AC

On Open Embeddings of Affine Spaces in Affine Varieties and the Jacobian Conjecture

Let $k$ be a field of \uline{characteristic $0$}. Our final goal is the following faded problem~: {\sf The Jacobian Conjecture $(JC_n)$~:} {\it If $f_1, \cdots, f_n$ are elements in a polynomial ring $k[X_1, \cdots, X_n]$ over $k$ such that $ \det(\partial f_i/ \partial X_j) $ is a nonzero constant, then $k[f_1, \cdots, f_n] = k[X_1, \cdots, X_n]$. } For this purpose, we consider the following Result~: \noindent {\sf Open Embeddings of Affine Spaces in Affine Varieties.} {\it Let $X$ be an irreducible $k$-affine variety with $\dim(X) = n$ and let $U$ be an open $\mathbb{C}$-subvariety of $X$ such that $U$ is isomorphic to $\mathbb{A}^n_k$. Then $X = U$.} This is effective for a more general result than our original objective $(JC_n)$. \noindent {\sf The Generalized Jacobian Conjecture $(GJC)$.} {\it Let $\varphi : X \rightarrow Y$ be an unramified morphism of normal $k$-affine varieties. If both $X$ and $Y$ are simply connected, then $\varphi$ is an isomorphism.} These results derived by $\mathbb{C}$-topological-method hold for $\mathbb{C}$ instead of $k$ by ``Lefschetz-principle''.

math.AC

On Bounding Problems on Totally Ordered Commutative Semi-Groups

The following is shown : Let $S=\{a_1,a_2,..,a_{2n}\}$ be a subset of a totally ordered commutative semi-group $(G,*,\leq)$ with $a_1\leq a_2\leq...\leq a_{2n}$. Provided that a system of $n$ $a_{i_k} * a_{j_k}\ (a_{i_k}, a_{j_k} \in G ;\ 1 \leq k \leq n)$, where all $2n$ elements in $S$ must be used, are less than an element $N\ (\in G)$, then $a_1*a_{2n}, a_2*a_{2n-1},..., a_n*a_{n+1}$ are all less than $N$. This may be called the Upper Bounding Case. Moreover in the same way, we shall treat also the Lower Bounding Case.

math.AC