Regular biorthogonal pairs and Psuedo-bosonic operators
  The first purpose of this paper is to show a method of constructing a regular biorthogonal pair based on the commutation rule: $ab-ba=I$ for a pair of operators $a$ and $b$ acting on a Hilbert space ${\cal H}$ with inner product $( \cdot | \cdot )$. Here, sequences $\{ \phi_{n} \}$ and $\{ \psi_{n} \}$ in a Hilbert space ${\cal H}$ are biorthogonal if $( \phi_{n} | \psi_{m})= \delta_{nm}$, $n,m=0,1, \cdots$, and they are regular if both $D_{\phi} \equiv Span \{ \phi_{n} \}$ and $D_{\psi} \equiv Span \{ \psi_{n} \}$ are dense in ${\cal H}$. Indeed, the assumption to construct the regular biorthogonal pair coincide with the definition of pseudo-bosons as originally given in Ref \cite{bagarello10}. Furthermore, we study the connections between the pseudo-bosonic operators $a, \; b, \; a^{\dagger}, \; b^{\dagger}$ and the pseudo-bosonic operators defined by a regular biorthogonal pair $(\{ \phi_{n} \}$, $\{ \psi_{n} \} )$ and an ONB $\mbox{ $e$}$ of ${\cal H}$ in appeared Ref \cite{hiroshi1}. The second purpose is to define and study the notion of ${\cal D}$-pseudo bosons in Ref \cite{bagarello13, bagarello2013} and give a method of constructing ${\cal D}$-pseudo bosons on some steps. Then it is shown that for any ONB $\mbox{ $e$}= \{ e_{n} \}$ in ${\cal H}$ and any operators $T$ and $T^{-1}$ in ${\cal L}^{\dagger} ( {\cal D})$, we may construct operators $A$ and $B$ satisfying ${\cal D}$-pseudo bosons, where ${\cal D}$ is a dense subspace in a Hilbert space ${\cal H}$ and ${\cal L}^{\dagger} ( {\cal D})$ the set of all linear operators $T$ from ${\cal D}$ to ${\cal D}$ such that $T^{\ast} {\cal D} \subset {\cal D}$, where $T^{\ast}$ is the adjoint of $T$. Finally, we give some physical examples of ${\cal D}$-pseudo bosons based on standard bosons by the method of constructing ${\cal D}$-pseudo bosons stated above. 
Hiroshi Inoue and Mayumi Takakura
math-ph math.MP