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- A Goto, K Kitamura, A Arai, T Shimizu
- Development, growth & differentiation
- 1999

As in other clitellate annelids, embryonic development in the oligochaete Tubifex is characterized by the generation of five bilateral pairs of teloblasts (designated M, N, O, P and Q), which serve as embryonic stem cells to produce germ bands on either side of the embryo. A large part of the tissues comprising body segments has been assigned to the… (More)

- A Nakamoto, A Arai, T Shimizu
- The International journal of developmental…
- 2000

Ectodermal segmentation in the oligochaete annelid Tubifex is a process of separation of 50-microm-wide blocks of cells from the initially continuous ectodermal germ band (GB), a cell sheet consisting of four bandlets of blast cells derived from ectoteloblasts (N, O, P and Q). In this study, using intracellular lineage tracers, we characterized the… (More)

- A Arai, A Nakamoto, T Shimizu
- Development
- 2001

In embryos of clitellate annelids (i.e. oligochaetes and leeches), four ectodermal teloblasts (ectoteloblasts N, O, P and Q) are generated on either side through a stereotyped sequence of cell divisions of a proteloblast, NOPQ. The four ectoteloblasts assume distinct fates and produce bandlets of smaller progeny cells, which join together to form an… (More)

- Asao Arai, Hiroyuki Kawano
- 2002

We consider, in an abstract form, a system of " quantum particles " coupled to a Bose field. It is shown that, under suitable hypotheses, the composed system can have a ground state even if the uncoupled particle system has no ground state.

- Asao Arai
- 2002

- Asao Arai
- 2008

We present new classes of time operators of a Hamiltonian H (a self-adjoint operator) with discrete eigenvalues which may be degenerate. Moreover we formulate necessary and sufficient conditions for H to have time operators, determining the general form of them. As corollaries, non-existence theorems of time operators for some classes of H are derived.

- Asao Arai
- 2000

We consider a model of quantum particles coupled to a massless quantum scalar eld, called the massless Nelson model, in a non-Fock representation of the time-zero elds which satisfy the canonical commutation relations. We show that the model has a ground state for all values of the coupling constant even in the case where no infrared cutoo is made. The… (More)

- Asao Arai, Masao Hirokawa, Fumio Hiroshima
- 2008

Regularities and higher order regularities of ground states of quantum field models are investigated through the fact that asymptotic annihilation operators vanish ground states. Moreover a sufficient condition for the absence of a ground state is given.

- Asao Arai
- 2008

Let (T, H) be a weak Weyl representation of the canonical commutation relation (CCR) with one degree of freedom. Namely T is a symmetric operator and H is a self-adjoint operator on a complex Hilbert space H satisfying the weak Weyl relation: For all t ∈ R (the set of real numbers), e −itH D(T) ⊂ D(T) (i is the imaginary unit and D(T) denotes the domain of… (More)

- Asao Arai, Masao Hirokawa
- 1999

We consider two kinds of stability (under a perturbation) of the ground state of a self-adjoint operator, being concerned with (i) the sector to which the ground state belongs and (ii) the uniqueness of the ground state. As an application to the Wigner-Weisskopf model which describes one mode fermion coupled to a quantum scalar eld, we prove in the massive… (More)