diff --git a/Changelog b/Changelog new file mode 100644 index 0000000..49d79c2 --- /dev/null +++ b/Changelog @@ -0,0 +1,8 @@ +v0.1: + + * Fixed: H_N is not compact, e^{-H_N} is. + + * Fixed: Removed confusing discussion of Anyons + + * Added: Discussion of compactness. + diff --git a/Jauslin_2023c.tex b/Jauslin_2023c.tex index 6eefc41..32beec0 100644 --- a/Jauslin_2023c.tex +++ b/Jauslin_2023c.tex @@ -296,10 +296,9 @@ and the other is that exchanging two particles leads to the wavefunction changin . \end{equation} Why are these the only two possibilities (in three dimensions)? -Mathematically, this comes from the fact that there are only two irreducible representations of the braid group in three dimensions. -More intuitively, if two particles are exchanged, the wavefunction should pick up a phase $e^{i\theta}$, but if I exchange the particles back, we should return to the original wavefunction, and thus pick up the phase $e^{-i\theta}$. -In three dimensions, there is no way of knowing which way is ``back'', so $e^{i\theta}=e^{-i\theta}$ and so $\theta$ is either $0$ or $\pi$. -(In two dimensions, one can know which way is ``back'', so there are other possible phases, but this is a discussion for another time). +Mathematically, this comes from the fact that there are only two irreducible representations of the permutation group in three dimensions. +More intuitively, if two particles are exchanged, the wavefunction should pick up a phase $e^{i\theta}$, but if I exchange the particles back, we should return to the original wavefunction, and thus $e^{2i\theta}=1$ and so $\theta$ is either $0$ or $\pi$. +(This is not the case in two dimensions, for reasons we will not elaborate here.) \bigskip \indent @@ -881,7 +880,7 @@ We will be interested in the {\it thermodynamic limit}, in which \frac NV=\rho \end{equation} with $\rho$ (the density) fixed. -In this case, the Hamiltonian\-~(\ref{Ham}) has pure-point spectrum (it is a compact operator). +In this case, the Hamiltonian\-~(\ref{Ham}) has pure-point spectrum (by Theorem\-~\ref{theo:compact_schrodinger}, $e^{-H_N}$ is compact, so it has discrete spectrum, and therefore so does $H_N$). Its lowest eigenvalue is denoted by $E_0$, and is called the {\it ground-state energy}. The corresponding eigenvector is denoted by $\psi_0$: \begin{equation} @@ -2201,7 +2200,7 @@ In this appendix we gather a few useful definitions and results from functional \endtheo \bigskip -\theoname{Theorem}{H\"older's inequality}\label{theo:holder} +\theoname{Theorem}{H\"older's inequality \cite[Problem 0.26]{Te14}}\label{theo:holder} If $f\in L_p(\Omega)$ and $g\in L_q(\Omega)$, then $fg\in L_r(\Omega)$ with \begin{equation} \frac1r=\frac1p+\frac 1q @@ -2253,18 +2252,72 @@ In this appendix we gather a few useful definitions and results from functional \bigskip \subsection{The Trotter product formula} -\theoname{Theorem}{Trotter product formula}\label{theo:trotter} - Given two operators $A$ and $B$ on a Banach space, +\theoname{Theorem}{Trotter product formula \cite[Theorem 5.11]{Te14}}\label{theo:trotter} + Given two operators $A$ and $B$ on a Hilbert space such that $A$, $B$ and $A+B$ are self-adjoint, and bounded from below, then for $t\geqslant 0$, \nopagebreakaftereq \begin{equation} - e^{A+B} + e^{-t(A+B)} = - \lim_{N\to\infty}\left(e^{\frac 1N A}e^{\frac1N B}\right)^N + \lim_{N\to\infty}\left(e^{-t\frac 1N A}e^{-t\frac1N B}\right)^N . \end{equation} \endtheo \restorepagebreakaftereq +\subsection{Compact and trace-class operators}\label{app:compact} +\theoname{Definition}{Trace} + Given a separable Hilbert space with an orthonormal basis $\{\varphi_i\}_{i\in\mathbb N}$, the trace of an operator $A$ is formally defined as + \begin{equation} + \mathrm{Tr}(A):=\sum_{i=0}^\infty \left<\varphi_i\right|A\left|\varphi_i\right> + . + \end{equation} + If this expression is finite, then $A$ is said to be trace-class. +\endtheo +\bigskip + +\theoname{Definition}{Compact operators} + The set of compact operators on a Hilbert space is the closure of the set of finite-rank operators. +\endtheo +\bigskip + +\theoname{Theorem}{\cite[Theorem 6.6]{Te14}} + If $A$ is a compact self-adjoint operator on a Hilbert space, then its spectrum consists of an at most countable set of eigenvalues. +\endtheo +\bigskip + +\theo{Theorem}\label{theo:trace_compact} + Trace class operators are compact. +\endtheo +\bigskip + +\theoname{Theorem}{\cite[Lemma 5.5]{Te14}}\label{theo:compact_ideal} + If $A$ is compact and $B$ is bounded, then $KA$ and $AK$ are compact. +\endtheo +\bigskip + +\theo{Theorem}\label{theo:compact_schrodinger} + Let $\Delta$ be the Laplacian on $L_2(\mathbb R^d/(L\mathbb Z)^d)$ (the same result holds for Dirichlet and Neumann boundary conditions as well). + For any function $v$ on $[0,L]^d$ that is bounded below, $e^{t(-\Delta+v)}$ is compact for any $t\geqslant 0$. +\endtheo +\bigskip + +\indent\underline{Proof}: + First of all, $e^{-t\Delta}$ is trace class: using the basis $e^{ikx}$ with $k\in(\frac{2\pi}L\mathbb Z)^d$ + \begin{equation} + \mathrm{Tr}e^{-t\Delta} + =\sum_{k\in(\frac{2\pi}L\mathbb Z)^d}e^{-tk^2}<\infty + . + \end{equation} + By Theorem\-~\ref{theo:trace_compact}, $e^{-t\Delta}$ is compact. + By the Trotter product formula, Theorem\-~\ref{theo:trotter}, + \begin{equation} + e^{-t\Delta+tv} + =\lim_{N\to\infty}(e^{-\frac tN\Delta}e^{\frac tNv})^N + \end{equation} + and by Theorem\-~\ref{theo:compact_ideal}, $(e^{-\frac tN\Delta}e^{\frac tNv})^N$ is compact. + Since the set of compact operators is closed, so is $e^{t(-\Delta+v)}$. +\qed + \subsection{Positivity preserving operators}\label{app:positivity_preserving} \theoname{Definition}{Positivity preserving operators}\label{def:positivity_preserving} An operator $A$ from a Banach space of real-valued functions $\mathcal B_1$ to another $\mathcal B_2$ is said to be {\it positivity preserving} if, for any $f\in\mathcal B_1$ such that $f(x)\geqslant0$, $Af(x)\geqslant 0$. @@ -2475,7 +2528,7 @@ In this appendix, we state some useful results from harmonic analysis. \restorepagebreakaftereq \bigskip -\theo{Theorem}\label{theo:harmonic} +\theoname{Theorem}{\cite[Theorem 9.4]{LL01}}\label{theo:harmonic} A function that is subharmonic on $A$ achieves its maximum on the boundary of $A$. A function that is superharmonic on $A$ achieves its minimum on the boundary of $A$. \endtheo @@ -3078,7 +3131,7 @@ This implies\-~(\ref{sol_softcore}). \solution{perron_frobenius} By Theorem\-~\ref{theo:schrodinger}, $e^{-tH_N}$ is positivity preserving. -In addition, $e^{-tH_N}$ is compact because $H_N$ is, and the spectrum of $e^{-tH_N}$ is $e^{-t\mathrm{spec}(H_N)}$, and the largest eigenvector of $e^{-tH_N}$ with the largest eigenvalue is the ground-state of $H_N$. +In addition, $e^{-tH_N}$ is compact by Theorem\-~\ref{theo:compact_schrodinger}, and the spectrum of $e^{-tH_N}$ is $e^{-t\ \mathrm{spec}(H_N)}$, and the largest eigenvector of $e^{-tH_N}$ with the largest eigenvalue is the ground-state of $H_N$. Finally, since $\mathrm{spec}(H_N)\geqslant 0$ (because $-\Delta\geqslant 0$ and $v\geqslant 0$, we can apply the Perron-Frobenius theorem, which implies that $\psi_0$ is unique and $\geqslant 0$. \bigskip diff --git a/bibliography/bibliography.tex b/bibliography/bibliography.tex index d8e5477..e8a830c 100644 --- a/bibliography/bibliography.tex +++ b/bibliography/bibliography.tex @@ -49,6 +49,8 @@ doi:{\tt\color{blue}\href{http://dx.doi.org/10.1103/PhysRev.130.2518}{10.1103/Ph \bibitem[LL64]{LL64}E.H. Lieb, W. Liniger - {\it Simplified Approach to the Ground-State Energy of an Imperfect Bose Gas. III. Application to the One-Dimensional Model}, Physical Review, volume\-~134, issue\-~2A, pages A312-A315, 1964,\par\penalty10000 doi:{\tt\color{blue}\href{http://dx.doi.org/10.1103/PhysRev.134.A312}{10.1103/PhysRev.134.A312}}.\par\medskip +\bibitem[LL01]{LL01}E.H. Lieb, M. Loss - {\it Analysis}, Second edition, Graduate studies in mathematics, Americal Mathematical Society, 2001.\par\medskip + \bibitem[LS64]{LS64}E.H. Lieb, A.Y. Sakakura - {\it Simplified Approach to the Ground-State Energy of an Imperfect Bose Gas. II. Charged Bose Gas at High Density}, Physical Review, volume\-~133, issue\-~4A, pages A899-A906, 1964,\par\penalty10000 doi:{\tt\color{blue}\href{http://dx.doi.org/10.1103/PhysRev.133.A899}{10.1103/PhysRev.133.A899}}.\par\medskip @@ -66,6 +68,8 @@ doi:{\tt\color{blue}\href{http://dx.doi.org/10.4171/90-2/40}{10.4171/90-2/40}}, \bibitem[Se11]{Se11}R. Seiringer - {\it The Excitation Spectrum for Weakly Interacting Bosons}, Communications in Mathematical Physics, volume\-~306, issue\-~2, pages\-~565-578, 2011,\par\penalty10000 doi:{\tt\color{blue}\href{http://dx.doi.org/10.1007/s00220-011-1261-6}{10.1007/s00220-011-1261-6}}, arxiv:{\tt\color{blue}\href{http://arxiv.org/abs/1008.5349}{1008.5349}}.\par\medskip +\bibitem[Te14]{Te14}G. Teschl - {\it Mathematical Methods in Quantum Mechanics With Applications to Schr\"odinger Operators}, Second Edition, Graduate Studies in Mathematics, volume\-~157, AMS, 2014.\par\medskip + \bibitem[YY09]{YY09}H. Yau, J. Yin - {\it The Second Order Upper Bound for the Ground Energy of a Bose Gas}, Journal of Statistical Physics, volume\-~136, issue\-~3, pages\-~453-503, 2009,\par\penalty10000 doi:{\tt\color{blue}\href{http://dx.doi.org/10.1007/s10955-009-9792-3}{10.1007/s10955-009-9792-3}}, arxiv:{\tt\color{blue}\href{http://arxiv.org/abs/0903.5347}{0903.5347}}.\par\medskip