Representation theory of SU(2)




In the study of the representation theory of Lie groups, the study of representations of SU(2) is fundamental to the study of representations of semisimple Lie groups. It is the first case of a Lie group that is both a compact group and a non-abelian group. The first condition implies the representation theory is discrete: representations are direct sums of a collection of basic irreducible representations (governed by the Peter–Weyl theorem). The second means that there will be irreducible representations in dimensions greater than 1.


SU(2) is the universal covering group of SO(3), and so its representation theory includes that of the latter, by dint of a surjective homomorphism to it. This underlies the significance of SU(2) for the description of non-relativistic spin in theoretical physics; see below for other physical and historical context.


As shown below, the finite-dimensional irreducible representations of SU(2) are indexed by a non-negative integer m{displaystyle m}m and have dimension m+1{displaystyle m+1}m+1. In the physics literature, the representations are labeled by the quantity l=m/2{displaystyle l=m/2}{displaystyle l=m/2}, where l{displaystyle l}l is then either an integer or a half-integer, and the dimension is 2l+1{displaystyle 2l+1}2l+1.




Contents






  • 1 Lie algebra representations


    • 1.1 Real and complexified Lie algebras


    • 1.2 Weights and the structure of the representation


    • 1.3 The Casimir element




  • 2 The group representations


    • 2.1 Action on polynomials


    • 2.2 The characters


    • 2.3 Relation to the representations of SO(3)




  • 3 Another approach


  • 4 Most important irreducible representations and their applications


  • 5 See also


  • 6 References





Lie algebra representations


The representations of the group are found by considering representations of su(2), the Lie algebra of SU(2). Since the group SU(2) is simply connected, every representation of its Lie algebra can be integrated to a group representation;[1] we will give an explicit construction of the representations at the group level below. A reference for this material is Section 4.6 of (Hall 2015).



Real and complexified Lie algebras


The real Lie algebra su(2) has a basis given by


u1=(0ii0)u2=(0−110)u3=(i00−i) ,{displaystyle u_{1}={begin{pmatrix}0&i\i&0end{pmatrix}}qquad u_{2}={begin{pmatrix}0&-1\1&0end{pmatrix}}qquad u_{3}={begin{pmatrix}i&0\0&-iend{pmatrix}}~,}u_{1}={begin{pmatrix}0&i\i&0end{pmatrix}}qquad u_{2}={begin{pmatrix}0&-1\1&0end{pmatrix}}qquad u_{3}={begin{pmatrix}i&0\0&-iend{pmatrix}}~,

which satisfy


[u1,u2]=2u3;[u2,u3]=2u1;[u3,u1]=2u2.{displaystyle [u_{1},u_{2}]=2u_{3};quad [u_{2},u_{3}]=2u_{1};quad [u_{3},u_{1}]=2u_{2}.}{displaystyle [u_{1},u_{2}]=2u_{3};quad [u_{2},u_{3}]=2u_{1};quad [u_{3},u_{1}]=2u_{2}.}

It is then convenient to pass to the complexified Lie algebra



su(2)+isu(2)=sl(2;C){displaystyle mathrm {su} (2)+imathrm {su} (2)=mathrm {sl} (2;mathbb {C} )}{displaystyle mathrm {su} (2)+imathrm {su} (2)=mathrm {sl} (2;mathbb {C} )}.

(Skew self-adjoint matrices with trace zero plus self-adjoint matrices with trace zero gives all matrices with trace zero.) As long as we are working with representations over C{displaystyle mathbb {C} }mathbb {C} this passage from real to complexified Lie algebra is harmless.[2] The reason for passing to the complexification is that it allows us to construct a nice basis of a type that does not exist in the real Lie algebra su(2).


The complexified Lie algebra is spanned by three elements X{displaystyle X}X, Y{displaystyle Y}Y, and H{displaystyle H}H, given by


H=1iu3;X=12i(u1−iu2);Y=12i(u1+iu2),{displaystyle H={frac {1}{i}}u_{3};quad X={frac {1}{2i}}(u_{1}-iu_{2});quad Y={frac {1}{2i}}(u_{1}+iu_{2}),}{displaystyle H={frac {1}{i}}u_{3};quad X={frac {1}{2i}}(u_{1}-iu_{2});quad Y={frac {1}{2i}}(u_{1}+iu_{2}),}

or, explicitly,


H=(100−1);X=(0100);Y=(0010) .{displaystyle H={begin{pmatrix}1&0\0&-1end{pmatrix}};quad X={begin{pmatrix}0&1\0&0end{pmatrix}};quad Y={begin{pmatrix}0&0\1&0end{pmatrix}}~.}{displaystyle H={begin{pmatrix}1&0\0&-1end{pmatrix}};quad X={begin{pmatrix}0&1\0&0end{pmatrix}};quad Y={begin{pmatrix}0&0\1&0end{pmatrix}}~.}

These satisfy the commutation relations



[H,X]=2X,[H,Y]=−2Y,[X,Y]=H{displaystyle [H,X]=2X,quad [H,Y]=-2Y,quad [X,Y]=H}{displaystyle [H,X]=2X,quad [H,Y]=-2Y,quad [X,Y]=H}.

Up to a factor of 2, the elements H{displaystyle H}H, X{displaystyle X}X and Y{displaystyle Y}Y may be identified with the angular momentum operators Jz{displaystyle J_{z}}J_{z}, J+{displaystyle J_{+}}{displaystyle J_{+}}, and J−{displaystyle J_{-}}J_{-}, respectively. The factor of 2 is a discrepancy between conventions in math and physics; we will attempt to mention both conventions in the results that follow.



Weights and the structure of the representation


In this setting, the eigenvalues for H{displaystyle H}H are referred to as the weights of the representation. The following elementary result[3] is a key step in the analysis. Suppose that v{displaystyle v}v is an eigenvector for H{displaystyle H}H with eigenvalue α{displaystyle alpha }alpha , that is, that H⋅v=αv{displaystyle Hcdot v=alpha v}{displaystyle Hcdot v=alpha v}. Then


H⋅(X⋅v)=(α+2)X⋅vH⋅(Y⋅v)=(α−2)Y⋅v{displaystyle {begin{aligned}Hcdot (Xcdot v)&=(alpha +2)Xcdot v\[3pt]Hcdot (Ycdot v)&=(alpha -2)Ycdot vend{aligned}}}{displaystyle {begin{aligned}Hcdot (Xcdot v)&=(alpha +2)Xcdot v\[3pt]Hcdot (Ycdot v)&=(alpha -2)Ycdot vend{aligned}}}

In other words, X⋅v{displaystyle Xcdot v}{displaystyle Xcdot v} is either the zero vector or an eigenvector for H{displaystyle H}H with eigenvalue α+2{displaystyle alpha +2}{displaystyle alpha +2} and Y⋅v{displaystyle Ycdot v}{displaystyle Ycdot v} is either zero or an eigenvector for H{displaystyle H}H with eigenvalue α−2{displaystyle alpha -2}{displaystyle alpha -2}. Thus, the operator X{displaystyle X}X acts as a raising operator, increasing the weight by 2, while Y{displaystyle Y}Y acts as a lowering operator.


Suppose now that V{displaystyle V}V is an irreducible, finite-dimensional representation of the complexified Lie algebra. Then H{displaystyle H}H can have only finitely many eigenvalues. In particular, there must be an eigenvalue λ∈C{displaystyle lambda in mathbb {C} }lambdainmathbb C with the property that λ+2{displaystyle lambda +2}{displaystyle lambda +2} is not an eigenvalue. Let v0{displaystyle v_{0}}v_{0} be an eigenvector for H{displaystyle H}H with eigenvalue λ{displaystyle lambda }lambda :



H⋅v0=λv0{displaystyle Hcdot v_{0}=lambda v_{0}}{displaystyle Hcdot v_{0}=lambda v_{0}}.

Then we must have



X⋅v0=0{displaystyle Xcdot v_{0}=0}{displaystyle Xcdot v_{0}=0},

or else the above identity would tell us that X⋅v0{displaystyle Xcdot v_{0}}{displaystyle Xcdot v_{0}} is an eigenvector with eigenvalue λ+2{displaystyle lambda +2}{displaystyle lambda +2}.


Now define a "chain" of vectors v0,v1,…{displaystyle v_{0},v_{1},ldots }{displaystyle v_{0},v_{1},ldots } by



vk=Yk⋅v0{displaystyle v_{k}=Y^{k}cdot v_{0}}{displaystyle v_{k}=Y^{k}cdot v_{0}}.

A simple argument by induction[4] then shows that


X⋅vk=k(λ−(k−1))vk−1{displaystyle Xcdot v_{k}=k(lambda -(k-1))v_{k-1}}{displaystyle Xcdot v_{k}=k(lambda -(k-1))v_{k-1}}

for all k=1,2,…{displaystyle k=1,2,ldots }k=1,2,ldots. Now, if vk{displaystyle v_{k}}v_{k} is not the zero vector, it is an eigenvector for H{displaystyle H}H with eigenvalue λ−2k{displaystyle lambda -2k}{displaystyle lambda -2k}. Since, again, H{displaystyle H}H has only finitely many eigenvectors, we conclude that vl{displaystyle v_{l}}v_{l} must be zero for some l{displaystyle l}l (and then vk=0{displaystyle v_{k}=0}{displaystyle v_{k}=0} for all k>l{displaystyle k>l}{displaystyle k>l}).


Let vm{displaystyle v_{m}}v_{m} be the last nonzero vector in the chain; that is, vm≠0{displaystyle v_{m}neq 0}{displaystyle v_{m}neq 0} but vm+1=0{displaystyle v_{m+1}=0}{displaystyle v_{m+1}=0}. Then of course X⋅vm+1=0{displaystyle Xcdot v_{m+1}=0}{displaystyle Xcdot v_{m+1}=0} and by the above identity with k=m+1{displaystyle k=m+1}{displaystyle k=m+1}, we have



0=X⋅vm+1=(m+1)(λ−m)vm{displaystyle 0=Xcdot v_{m+1}=(m+1)(lambda -m)v_{m}}{displaystyle 0=Xcdot v_{m+1}=(m+1)(lambda -m)v_{m}}.

Since m+1{displaystyle m+1}m+1 is at least one and vm≠0{displaystyle v_{m}neq 0}{displaystyle v_{m}neq 0}, we conclude that λ{displaystyle lambda }lambda must be equal to the non-negative integer m{displaystyle m}m.


We thus obtain a chain of m+1{displaystyle m+1}m+1 vectors v0,…,vm{displaystyle v_{0},ldots ,v_{m}}{displaystyle v_{0},ldots ,v_{m}} such that Y{displaystyle Y}Y acts as


Y⋅vm=0;Y⋅vk=vk+1(k<m){displaystyle Ycdot v_{m}=0;quad Ycdot v_{k}=v_{k+1}quad (k<m)}{displaystyle Ycdot v_{m}=0;quad Ycdot v_{k}=v_{k+1}quad (k<m)}

and X{displaystyle X}X acts as


X⋅v0=0,X⋅vk=k(m−(k−1))vk−1(k>0){displaystyle Xcdot v_{0}=0,quad Xcdot v_{k}=k(m-(k-1))v_{k-1}quad (k>0)}{displaystyle Xcdot v_{0}=0,quad Xcdot v_{k}=k(m-(k-1))v_{k-1}quad (k>0)}

and H{displaystyle H}H acts as



H⋅vk=(m−2k)vk{displaystyle Hcdot v_{k}=(m-2k)v_{k}}{displaystyle Hcdot v_{k}=(m-2k)v_{k}}.

(We have replaced λ{displaystyle lambda }lambda with its currently known value of m{displaystyle m}m in the above formulas.)


Since the vectors vk{displaystyle v_{k}}v_{k} are eigenvectors for H{displaystyle H}H with distinct eigenvalues, they must be linearly independent. Furthermore, the span of v0,…,vm{displaystyle v_{0},ldots ,v_{m}}{displaystyle v_{0},ldots ,v_{m}} is clearly invariant under the action of the complexified Lie algebra. Since V{displaystyle V}V is assumed irreducible, this span must be all of V{displaystyle V}V. We thus obtain a complete description of what an irreducible representation must look like; that is, a basis for the space and a complete description of how the generators of the Lie algebra act. Conversely, for any m≥0{displaystyle mgeq 0}mgeq 0 we can construct a representation by simply using the above formulas and checking that the commutation relations hold. This representation can then be shown to be irreducible.[5]


Conclusion: For each non-negative integer m{displaystyle m}m, there is a unique irreducible representation with highest weight m{displaystyle m}m. Each irreducible representation is equivalent to one of these. The representation with highest weight m{displaystyle m}m has dimension m+1{displaystyle m+1}m+1 with weights m,m−2,…,−(m−2),−m{displaystyle m,m-2,ldots ,-(m-2),-m}{displaystyle m,m-2,ldots ,-(m-2),-m}, each having multiplicity one.



The Casimir element


We now introduce the (quadratic) Casimir element, C{displaystyle C}C given by



C=−(u12+u22+u32){displaystyle C=-(u_{1}^{2}+u_{2}^{2}+u_{3}^{2})}{displaystyle C=-(u_{1}^{2}+u_{2}^{2}+u_{3}^{2})}.

We can view C{displaystyle C}C as an element of the universal enveloping algebra or as an operator in each irreducible representation. Viewing C{displaystyle C}C as an operator on the representation with highest weight m{displaystyle m}m, we may easily compute that C{displaystyle C}C commutes with each ui{displaystyle u_{i}}u_{i}. Thus, by Schur's lemma, C{displaystyle C}C acts as a scalar multiple cm{displaystyle c_{m}}c_{m} of the identity for each m{displaystyle m}m. We can write C{displaystyle C}C in terms of the H,X,Y{displaystyle {H,X,Y}}{displaystyle {H,X,Y}} basis as follows:



C=(X+Y)2−(−X+Y)2+H2{displaystyle C=(X+Y)^{2}-(-X+Y)^{2}+H^{2}}{displaystyle C=(X+Y)^{2}-(-X+Y)^{2}+H^{2}},

which simplifies to



C=4YX+H2+2H{displaystyle C=4YX+H^{2}+2H}{displaystyle C=4YX+H^{2}+2H}.

The eigenvalue of C{displaystyle C}C in the representation with highest weight m{displaystyle m}m can be computed by applying C{displaystyle C}C to the highest weight vector, which is annihilated by X{displaystyle X}X. Thus, we get



cm=m2+2m=m(m+2){displaystyle c_{m}=m^{2}+2m=m(m+2)}{displaystyle c_{m}=m^{2}+2m=m(m+2)}.

In the physics literature, the Casimir is normalized as C′=C/4{displaystyle C'=C/4}{displaystyle C'=C/4}. Labeling things in terms of l=m/2{displaystyle l=m/2}{displaystyle l=m/2}, the eigenvalue dl{displaystyle d_{l}}{displaystyle d_{l}} of C′{displaystyle C'}C' is then computed as



dl=14(2l)(2l+2)=l(l+1){displaystyle d_{l}={frac {1}{4}}(2l)(2l+2)=l(l+1)}{displaystyle d_{l}={frac {1}{4}}(2l)(2l+2)=l(l+1)}.


The group representations



Action on polynomials


Since SU(2) is simply connected, a general result shows that every representation of its (complexified) Lie algebra gives rise to a representation of SU(2) itself. It is desirable, however, to give an explicit realization of the representations at the group level. The group representations can be realized on spaces of polynomials in two complex variables.[6] That is, for each non-negative integer m{displaystyle m}m, we let Vm{displaystyle V_{m}}V_{m} denote the space of homogeneous polynomials of degree m{displaystyle m}m in two complex variables. Then the dimension of Vm{displaystyle V_{m}}V_{m} is m+1{displaystyle m+1}m+1. There is a natural action of SU(2) on each Vm{displaystyle V_{m}}V_{m}, given by



[U⋅p](z)=p(U−1z),z∈C2,U∈SU(2){displaystyle [Ucdot p](z)=p(U^{-1}z),quad zin mathbb {C} ^{2},,Uin mathrm {SU} (2)}{displaystyle [Ucdot p](z)=p(U^{-1}z),quad zin mathbb {C} ^{2},,Uin mathrm {SU} (2)}.

The associated Lie algebra representation is simply the one described in the previous section. (See here for an explicit formula for the action of the Lie algebra on the space of polynomials.)



The characters


The character of a representation Π:G→GL(V){displaystyle Pi :Grightarrow mathrm {GL} (V)}{displaystyle Pi :Grightarrow mathrm {GL} (V)} is the function X:G→C{displaystyle mathrm {X} :Grightarrow mathbb {C} }{displaystyle mathrm {X} :Grightarrow mathbb {C} } given by



X(g)=trace(Π(g)){displaystyle mathrm {X} (g)=mathrm {trace} (Pi (g))}{displaystyle mathrm {X} (g)=mathrm {trace} (Pi (g))}.

Characters plays an important role in the representation theory of compact groups. The character is easily seen to be a class function, that is, invariant under conjugation.


In the SU(2) case, the fact that the character is a class function means it is determined by its value on the maximal torus T{displaystyle T}T consisting of the diagonal matrices in SU(2). Since the irreducible representation with highest weight m{displaystyle m}m has weights m,m−2,…−(m−2),−m{displaystyle m,m-2,ldots -(m-2),-m}{displaystyle m,m-2,ldots -(m-2),-m}, it is easy to see that the associated character satisfies


X((eiθ00e−iθ))=eimθ+ei(m−2)θ+⋯e−i(m−2)θ+e−imθ.{displaystyle mathrm {X} left({begin{pmatrix}e^{itheta }&0\0&e^{-itheta }end{pmatrix}}right)=e^{imtheta }+e^{i(m-2)theta }+cdots e^{-i(m-2)theta }+e^{-imtheta }.}{displaystyle mathrm {X} left({begin{pmatrix}e^{itheta }&0\0&e^{-itheta }end{pmatrix}}right)=e^{imtheta }+e^{i(m-2)theta }+cdots e^{-i(m-2)theta }+e^{-imtheta }.}

This expression is a finite geometric series that can be simplified to


X((eiθ00e−iθ))=sin((m+1)θ)sin(θ).{displaystyle mathrm {X} left({begin{pmatrix}e^{itheta }&0\0&e^{-itheta }end{pmatrix}}right)={frac {mathrm {sin} ((m+1)theta )}{mathrm {sin} (theta )}}.}{displaystyle mathrm {X} left({begin{pmatrix}e^{itheta }&0\0&e^{-itheta }end{pmatrix}}right)={frac {mathrm {sin} ((m+1)theta )}{mathrm {sin} (theta )}}.}

This last expression is just the statement of the Weyl character formula for the SU(2) case.[7]


Actually, following Weyl's original analysis of the representation theory of compact groups, one can classify the representations entirely from the group perspective, without using Lie algebra representations at all. In this approach, the Weyl character formula plays an essential part in the classification, along with the Peter–Weyl theorem. The SU(2) case of this story is described here.



Relation to the representations of SO(3)


See also: Rotation group SO(3) § Connection between SO(3) and SU(2), and Projective representation

Note that either all of the weights of the representation are even (if m{displaystyle m}m is even) or all of the weights are odd (if m{displaystyle m}m is odd). In physical terms, this distinction is important: The representations with even weights correspond to ordinary representations of the rotation group SO(3).[8] By contrast, the representations with odd weights correspond to double-valued (spinorial) representation of SO(3), also known as projective representations.


In the physics conventions, m{displaystyle m}m being even corresponds to l{displaystyle l}l being an integer while m{displaystyle m}m being odd corresponds to l{displaystyle l}l being a half-integer. These two cases are described as integer spin and half-integer spin, respectively. The representations with odd, positive values of m{displaystyle m}m are faithful representations of SU(2), while the representations of SU(2) with non-negative, even m{displaystyle m}m are not faithful.[9]



Another approach


See under the example for Borel–Weil–Bott theorem.



Most important irreducible representations and their applications


Main article: Rotation group SO(3) § A note on Lie algebra

As stated above, representations of SU(2) describe non-relativistic spin, due to being a double covering of the rotation group of Euclidean 3-space. Relativistic spin is described by the representation theory of SL2(C), a supergroup of SU(2), which in a similar way covers SO+(1;3), the relativistic version of the rotation group. SU(2) symmetry also supports concepts of isobaric spin and weak isospin, collectively known as isospin.


The representation with m=1{displaystyle m=1}m=1 (i.e., l=1/2{displaystyle l=1/2}{displaystyle l=1/2} in the physics convention) is the 2 representation, the fundamental representation of SU(2). When an element of SU(2) is written as a complex 2 × 2 matrix, it is simply a multiplication of column 2-vectors. It is known in physics as the spin-½ and, historically, as the multiplication of quaternions (more precisely, multiplication by a unit quaternion). This representation can also be viewed as a double-valued projective representation of the rotation group SO(3).


The representation with m=2{displaystyle m=2}m=2 (i.e., l=1{displaystyle l=1}l=1) is the 3 representation, the adjoint representation. It describes 3-d rotations, the standard representation of SO(3), so real numbers are sufficient for it. Physicists use it for the description of massive spin-1 particles, such as vector mesons, but its importance for spin theory is much higher because it anchors spin states to the geometry of the physical 3-space.
This representation emerged simultaneously with the 2 when William Rowan Hamilton introduced versors, his term for elements of SU(2). Note that Hamilton did not use standard group theory terminology since his work preceded Lie group developments.


The m=3{displaystyle m=3}m=3 (i.e. l=3/2{displaystyle l=3/2}{displaystyle l=3/2}) representation is used in particle physics for certain baryons, such as the Δ.



See also



  • Rotation operator (vector space)

  • Rotation operator (quantum mechanics)

  • Representation theory of SO(3)

  • Connection between SO(3) and SU(2)

  • representation theory of SL2(R)

  • Electroweak interaction

  • Rotation group SO(3) § A note on Lie algebra



References





  1. ^ Hall 2015 Theorem 5.6


  2. ^ Hall 2015 Section 3.6


  3. ^ Hall 2015 Lemma 4.33


  4. ^ Hall 2015 Equation (4.15)


  5. ^ Hall 2015 proof of Proposition 4.11


  6. ^ Hall 2015 Section 4.2


  7. ^ Hall 2015 Example 12.23


  8. ^ Hall 2015 Section 4.7


  9. ^ Ma, Zhong-Qi (2007-11-28). Group Theory for Physicists. World Scientific Publishing Company. p. 120. ISBN 9789813101487..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"""""""'""'"}.mw-parser-output .citation .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}





  • Hall, Brian C. (2015), Lie Groups, Lie Algebras, and Representations: An Elementary Introduction, Graduate Texts in Mathematics, 222 (2nd ed.), Springer, ISBN 978-3319134666

  • Gerard 't Hooft (2007), Lie groups in Physics, Chapter 5 "Ladder operators"


  • Iachello, Francesco (2006), Lie Algebras and Applications, Lecture Notes in Physics, 708, Springer, ISBN 3540362363




-

這個網誌中的熱門文章

Xamarin.form Move up view when keyboard appear

圖片
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ height:90px;width:728px;box-sizing:border-box; } 0 I'm trying to build a Chat app UI, the idea of the Layout was pretty simple: When the input bar is focused, keyboard show up and "push" up the chat bar, as it's a grid, the ListView will resize to fit the screen: I update the input bar's margin to "push" it up: NSValue result = (NSValue)args.Notification.UserInfo.ObjectForKey(new NSString(UIKeyboard.FrameEndUserInfoKey)); CGSize keyboardSize = result.RectangleFValue.Size; if (Element != null){ Element.Margin = new Thickness(0, 0, 0,keyboardSize.Height); //push the entry up to keyboard height when keyboard is activated } And this is ...
閱讀完整內容

Post-Redirect-Get with Spring WebFlux and Thymeleaf

圖片
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ height:90px;width:728px;box-sizing:border-box; } 2 Can someone explain to me how to implement the post-redirect-get pattern in Spring WebFlux and Thymeleaf? What subscribes on the database save method? @GetMapping("/register") public String showRegisterForm(Model model) { model.addAttribute("user", new User()); return "register"; } @PostMapping public String processRegisterForm(@Valid User user, BindingResult bindingResult) { if (bindingResult.hasErrors()) { return "register"; } else { userRepository.save(user); //what subscribes on this? //how to redirect on e.g. "/login"? ...
閱讀完整內容

Anylogic : not able to use stopDelay()

圖片
-2 I am building a railway station model. I want a train to depart when I click a button. I want to call stopDelay() for that but I am not sure how to do that and from which place. anylogic share | improve this question asked Nov 23 '18 at 10:12 Anupam Singh Anupam Singh 1 Hi Anupam, welcome to SOF. Please check how to ask good questions, see link below. Your question is...
閱讀完整內容