# K-boy

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bio website location Nanjing, China age 27 member for 1 year, 9 months seen Nov 18 at 14:41 profile views 1,638

I'm a graduate student studying theoretical condensed matter physics in Nanjing university, China.

My current interest is in the topics of symmetry, gauge theory, and topology in condensed matter theory.

# 478 Actions

 Nov18 revised Can we use combined symmetry to simplify the calculation of algebraic PSGs? added 42 characters in body Nov18 asked Can we use combined symmetry to simplify the calculation of algebraic PSGs? Nov14 awarded Popular Question Nov5 comment Symmetry, gauge, and projective symmetry group (PSG)? Another simple formula: If $G_UU\in PSG(H)$, then $G_{U^{-1}}=U^{-1}G_U^{-1}U$ such that $G_{U^{-1}}U^{-1}\in PSG(H)$. Nov5 revised Symmetry, gauge, and projective symmetry group (PSG)? deleted 132 characters in body Nov4 revised Symmetry, gauge, and projective symmetry group (PSG)? added 424 characters in body Nov3 asked Symmetry, gauge, and projective symmetry group (PSG)? Oct26 comment Simple questions on the symmetric eigenstate and time-reversal (TR) breaking eigenstate? @luming Thanks for your comment. $\psi_1\pm \psi_2\neq 0$ because $\psi_1$ and $\psi_2$ are two linear independent eigenstates. Oct25 asked Simple questions on the symmetric eigenstate and time-reversal (TR) breaking eigenstate? Sep30 awarded Explainer Sep24 awarded Autobiographer Sep19 comment Is there a critical order of the Abelian gauge theory in (2+1)D @Everett You Dear Everett, I have a naive comment here. I don't know whether the group $Z_n$ has a well-defined limit $\lim_{n\rightarrow \infty}Z_n$ at the mathematical level. For example, if $Z_n\equiv \left \{exp(il\frac{2\pi}{n});l=1,2,...,n\right \}$, then it seems that $\lim_{n\rightarrow \infty}Z_n=U(1)$; while, if $Z_n\equiv \left \{ (-\frac{n}{2})^*,(-\frac{n}{2}+1)^*,...,0^*,1^*,2^*,...,(\frac{n}{2}-1)^*\right \}$($n$ is even for example), where $m^*\equiv \left \{ m,m\pm n,m\pm 2n,...\right \}$, then it seems that $\lim_{n\rightarrow \infty}Z_n=Z$($Z$ represents all the integers). Sep19 comment Some questions about anyons? I'm sorry that I just saw your answer, thank you. I can not understand your explanation immediately, but I just found a related article. Aug6 comment Naive questions on the concept of effective Lagrangian and equations of motion? I think I can answer my Q3 now: Classically, without the path integral method, express $x$ in terms of $E$ by solving the Euler-Lagrangian equation for $x$ derived from the total Lagrangian $L$, then substitute $x$ back into $L$, and we will obtain the same effective Lagrangian $L_{eff}$ containing only $E$. Aug4 comment Naive questions on the concept of effective Lagrangian and equations of motion? And mathematically speaking, $OE=0$ does not always give the correct equation of motion. For example, let's take $O=\partial _t$, then $OE=0$ gives the equation $\partial _tE=0$, while the Euler-Lagrangian equation gives a trivial equation $\partial _tE=\partial _tE$. And here $O=\partial _t$ is NOT positive. Aug4 comment Naive questions on the concept of effective Lagrangian and equations of motion? If $O$ is not positive, then there exists a function $f(t)$ such that $\int dtfOf<0=\int dtEOE$, where $OE=0$, which means that the solution $E$ does NOT correspond to the minimal action $S_{eff}$. Aug3 comment Naive questions on the concept of effective Lagrangian and equations of motion? :Inspired by your comment, if there exists a least action principle for the effective action $S_{eff}=\int dtL_{eff}$, and further the operator $O$ is assumed to be positive, then $OE=0$ may be one possible solution. So what's your opinion? Aug2 comment Naive questions on the concept of effective Lagrangian and equations of motion? ♦ Yes. And I wonder whether we could define an effective Lagrangian containing only the electric field $E$ from the classical point of view? Thanks. Aug2 revised Naive questions on the concept of effective Lagrangian and equations of motion? edited tags Aug2 comment Naive questions on the concept of effective Lagrangian and equations of motion? ♦ Thanks for your answer. You have given the classical equations of motion using normal coordinates. But I'm not interested in how to solve the classical harmonic oscillators, instead, here what I concerned is the concept of effective Lagrangian, either quantum version or classical version.