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For the adjoint covariant derivative $D_i={\partial }_i+[A_i,\cdot ]$, the gauge curvature isThe matrix-valued wedge product gives one commutator in each $d{x}^i\wedge d{x}^j$ coefficient; $[D_i,D_j]\Phi =[F_{ij},\Phi ]$ follows directly. The given trace defines a real positive inner product on the anti-Hermitian algebra $\mathfrak{su}(2)$.

The additional terms in $⟨D_j\Phi ,\Psi ⟩+⟨\Phi ,D_j\Psi ⟩$, compared with ordinary differentiation, areThus trace cyclicity proves the compatibility identityWith compactly supported variations, this gives covariant integration by parts without boundary contributions.

Use $F_{ij}={ϵ}_{ijk}{B}_k$, so $|B{|}^2=\frac{1}{2}{\sum }_{i,j}|F_{ij}{|}^2$. Write the independent variations as $\delta A_i=a_i$ and $\delta \Phi =\chi$. The curvature and Higgs derivative variations areThe variation of the Yang-Mills theory magnetic energy isFor the Higgs field derivative energy, trace cyclicity also gives $⟨X,[a,\Phi ]⟩=⟨[\Phi ,X],a⟩$. HenceArbitrariness of the variations proves the Euler-Lagrange field equationsThe gauge equation can equivalently be written $(D\times B{)}_j+[\Phi ,D_j\Phi ]=0$, where $(D\times B{)}_j={ϵ}_{jik}{D}_i{B}_k$. This equivalent form also checks the sign of the commutator source.

Now suppose the positive-sign Bogomolny equations $B_i=D_i\Phi$ hold. The gauge-theory Bianchi identity $D_i{F}_{jk}+D_j{F}_{ki}+D_k{F}_{ij}=0$ implies ${\sum }_i{D}_i{B}_i=0$, and therefore ${\sum }_i{D}_i{D}_i\Phi =0$. For the gauge equation,Consequently $(D\times B{)}_i+[\Phi ,D_i\Phi ]=[B_i,\Phi ]+[\Phi ,B_i]=0$. Thus the first-order Bogomolny equations imply both second-order field equations. The negative-sign convention works as well, with both occurrences of that sign tracked consistently.

Finally differentiate the norm twice, using the trace compatibility identity at each step:The second term vanishes, and $|B{|}^2=|D\Phi {|}^2$ for the Bogomolny equations. Thus the Higgs norm identity for a Bogomolny monopole givesThere is no extra factor 2 multiplying $v$: the printed energy density already contains both equal terms without an overall $1/2$.

A BPS state is a state whose mass saturates a lower bound set by conserved charges in the Super-Poincaré algebra. Saturation leaves some linear combinations of supercharges unbroken and produces a shortened massive supermultiplet. Both statements follow from the positivity of the physical Hilbert-space norm, rather than from an assumed classical field configuration.

For four-dimensional extended supersymmetry, choose the normalizationThe conjugate algebra is obtained by Hermitian conjugation. The antisymmetry of the central charge in supersymmetry matrix follows because the anticommutator is symmetric in the combined labels $(\alpha ,A)$ while ${ϵ}_{\alpha \beta }$ is antisymmetric. These central charges commute with momentum and supercharges, so their eigenvalues are fixed in an irreducible massive representation. In the rest frame, $P_{\mu }=(M,0,0,0)$ and $\{Q_{\alpha }^A,(Q_{\beta }^B{)}^{†}\}=2M{\delta }^{AB}{\delta }_{\alpha \beta }$.

For any operator $S$ and physical state $|v⟩$,Apply this to $S_{\alpha }^A=Q_{\alpha }^A-{ϵ}_{\alpha \beta }{U}^{AB}(Q_{\beta }^B{)}^{†}$, where $U$ is unitary. This is the rest-frame version of the positive operator constructed from $Q-\Gamma$. Summing over spinor and internal labels givesThe two diagonal terms each contribute $4\mathcal{N}M$, because $U^{†}U=I$; each mixed term contains ${\sum }_{\alpha \beta }{ϵ}_{\alpha \beta }^2=2$. For $\mathcal{N}=2$, write $Z^{12}=|Z|e^{i\phi }$ and choose $U=e^{i\phi }\left(\begin{array}{cc}0& 1\\ -1& 0\end{array}\right)$. Then $\mathcal{H}=16M-8|Z|$, so $M\ge |Z|/2$. The factor of one-half reflects the stated convention with $Z$, rather than $2Z$, in the equal-chirality anticommutator.

To obtain the sharp general bound and exhibit the shortening, use unitary skew-diagonalization of an antisymmetric matrix to put $Z$ into two-by-two blocks $Z_r\left(\begin{array}{cc}0& 1\\ -1& 0\end{array}\right)$, with a possible zero row and column for odd $\mathcal{N}$. In one block, relabel its two internal indices $1,2$ and write $Z_r=|Z_r|e^{i{\phi }_r}$. DefineDirect substitution, using ${ϵ}_{12}=1$, givesand all annihilator-annihilator anticommutators vanish. The minus operators must have nonnegative norm in every block, hence the BPS bound in supersymmetry isEquivalently, defining $z_r=Z_r/2$ gives $M\ge {\max }_r|z_r|$. Applying the positive $Q-\Gamma$ construction to each block gives the same bound; applying only its full trace need not isolate the largest block. For $M>0$ and no central charge saturation, each operator can be normalized by $\sqrt{2M\pm |Z_r|}$ to an ordinary fermionic annihilation operator. The unmatched internal index, if any, contributes two more such operators.

There are consequently $2\mathcal{N}$ complex creation operators in a generic massive supermultiplet. Starting with a spin-$j$ Clifford vacuum, each creation operator can be occupied zero or one times, so the long representation contains $(2j+1)2^{2\mathcal{N}}$ states. If precisely $k$ central-charge blocks saturate $2M=|Z_r|$, then the two $a_{r,i}^{-}$ and their adjoints have zero anticommutator. The norm identity forces each of them to annihilate every physical state in that representation. Removing these $2k$ complex oscillators givesThus a massive $\mathcal{N}=2$ BPS supermultiplet is half-BPS. For $j=0$, its four states consist of two spin-zero states and a spin-one-half doublet, instead of the sixteen states of a long representation. A charged representation generally needs its conjugate for CPT completion of a supermultiplet, doubling this four-state count; this yields the eight states of a full charged hypermultiplet. For $\mathcal{N}=4$, one saturated block gives a quarter-BPS representation and two give a half-BPS representation. This rest-frame argument assumes $M>0$; massless multiplets follow instead from the null-momentum algebra.

The same bound has a geometric realization for solitons. Consider a static magnetic monopole in an $SU(2)$ gauge theory with an adjoint Higgs field ${\varphi }^a$, asymptotic magnitude $v$, vanishing scalar potential and magnetic field $B_i^a$. For magnetic charge $n_m$, the energy can be writtenThe second equality uses the covariant magnetic Bianchi identity to turn the cross term into the surface integral $\oint {\varphi }^a{B}_i^{a}d{S}_i$. Choosing its sign proves the Bogomolny bound $\boxed{E\ge 4\pi v|n_m|/g_{\mathrm{YM}}}$. Equality gives the first-order Bogomolny equations $B_i^a=\pm D_i{\varphi }^a$. In a suitable $\mathcal{N}=2$ supersymmetric embedding, the surface charge is the appropriate central charge and the soliton obeys precisely the algebraic BPS bound in supersymmetry. Its fermionic variation vanishes for the preserved supersymmetry parameters, while the broken supercharges supply its fermionic zero modes.

Shortening is useful because an isolated short multiplet cannot continuously acquire the additional states of a long representation. While the state exists and stays short, its exact mass is determined by the exact central charge, so masses of strongly interacting solitons can be found without computing a generic interacting-particle self-energy. This does not mean that its numerical mass is independent of coupling or vacuum expectation values: those can enter the central charge itself. Supersymmetric indices and the reduced fermionic state content provide ways to count protected states, subject to possible recombination or continuum effects.

For example, in rank-one Seiberg–Witten theory the electric and magnetic charges $(n_e,n_m)$ enter through periods $a,a_D$. In the normalization above, $Z^{12}=2\sqrt{2}(n_{e}a+n_m{a}_D)$, so a BPS state has $\boxed{M=\sqrt{2}|n_{e}a+n_m{a}_D|}$. A magnetic monopole can become massless where $a_D=0$, even though the original electric description is strongly coupled. Including this light magnetic hypermultiplet supplies a useful local description there; states with both electric and magnetic charge are dyons. Thus the protected spectrum identifies important singularities of the vacuum space and tests electric-magnetic dual descriptions.

The charge bound also controls stability. For any proposed decay with additive complex charges $Z={\sum }_i{Z}_i$, each daughter satisfies $M_i\ge |Z_i|/2$. A BPS parent satisfiesEnergy conservation permits decay only if both inequalities are equalities: all daughters must be BPS and their nonzero complex charges must have the same phase. With nonaligned phases the parent lies strictly below the multiparticle threshold. A wall of marginal stability is a locus where such phases align, and a short bound state can disappear across it. Therefore BPS saturation protects a mass-charge relation and short representation, while stability and existence still depend on the spectrum and parameters.

Put $h=|\Phi {|}^2$, $D_i={\partial }_i-i{A}_i$, $j_i=\mathrm{Im}(\bar{\Phi }{D}_i\Phi )$, and $W(h)=h(1-h{)}^2/4$. Because the inverse-density Abelian Higgs model contains $B^2/h$, its finite-energy admissible class matters at zeros of the Higgs field. First derive the formal equations on the open set where $h>0$; on the first-order solutions below the potentially singular quotients extend across the zeros.

For variations $\psi =\delta \Phi$ and $a_i=\delta A_i$,Covariant integration by parts in the scalar term, and ordinary integration by parts in the magnetic term, givewhere ${ϵ}_{12}=1$. HereThus the Euler-Lagrange field equations areEquivalently the gauge equation is ${\partial }_j(F_{ji}/h)+j_i=0$. The current sign follows from the explicit choice $D=\partial -iA$.

For the Bogomolny bound for the inverse-density Abelian Higgs model, computeusing $[D_1,D_2]\Phi =-iB\Phi$. Expanding the first square givessoAlso,The current boundary integral vanishes by the assumed rapid decay. Combining the identities therefore givesFor fixed positive flux the minimum is $\boxed{V_{\min }=\pi N}$, attained when the Bogomolny equations areThe quotient at a zero is understood through the finite-energy limit, not by dividing a literal nonzero numerator by zero.

These equations imply the second-order equations directly. Applying $D_1-i{D}_2$ to the first gives $(D_i{D}_i+B)\Phi =0$. On the second equation,which is exactly the scalar equation. The first equation also implies $j_1=-\frac{1}{2}{\partial }_{2}h$ and $j_2=\frac{1}{2}{\partial }_{1}h$. Since $B/h=(1-h)/2$, the gauge equations follow too. All these coefficients are regular across zeros for smooth first-order fields.

For the scalar reduction, write $\Phi =e^{u/2+i\chi }$ away from its zeros. Separating the real and imaginary parts of the first Bogomolny equation givesThus, with the oriented planar Hodge star $\ast d{x}^1=d{x}^2$, $\ast d{x}^2=-d{x}^1$,away from the zeros. A zero $p_j$ of multiplicity $n_j$ has $u=2n_j\log |x-p_j|+\text{smooth}$ and phase winding $2\pi n_j$. The multiplicities are positive: locally the first-order equation is a covariant Cauchy–Riemann equation, and a nonvanishing integrating factor turns its solutions into holomorphic functions. Distributionally,Consequently the logarithmic equation for inverse-density Abelian Higgs vortices isThe delta terms are essential for multi-vortex reconstruction; omitting them would lose the specified zeros and flux.

Here is an explicit regular reconstruction, which also removes all artificial singularities. Given distinct locations $p_j$ and integer multiplicities, putSince $\Delta u_0=4\pi {\sum }_j{n}_j{\delta }_{p_j}-g$, solving the smooth nonlinear elliptic problem for $v=u-u_0$ amounts toLet $z=x^1+i{x}^2$ and let $z_j$ represent $p_j$. A smooth solution with the topological decay reconstructsThese formulas are smooth at every prescribed zero and agree with the phase formula off the zeros. Taking a curl and using the equation for $v$ verifies $B=e^u(1-e^u)/2$; substitution verifies the other first-order equation. At infinity the phase winds $N$ times, producing flux $2\pi N$. Thus multi-vortex construction reduces to solving one scalar elliptic boundary-value problem for specified positions and multiplicities, followed by these explicit formulas.

For the topological branch, $u\le 0$: a positive interior maximum would have $\Delta u\le 0$, whereas the equation there gives $\Delta u=e^u(e^u-1)>0$. At infinity, the linearized scalar equation is $\Delta u-u=0$. Comparison barriers and elliptic estimates give the decaying tail for regular topological solutions, and hence the required decay of $B,D\Phi$ and $1-h$. The nonlinear elliptic problem determines $v$; the reconstruction formulas do not constitute an arbitrary superposition of single-vortex profiles.

The degree of a map between oriented manifolds measures how many times the domain covers the target, with signs recording the local local orientation of a manifold. Let $M,N$ be connected, oriented closed manifolds of the same dimension $n>0$. A continuous map $f:M\to N$ acts on top-dimensional homology bywhere $[M],[N]$ are their fundamental classes. Connectedness and the choices of local orientation of a manifold identify $H_n(N;\mathbb{Z})$ with $\mathbb{Z}$. Reversing the orientation of either manifold changes the sign; reversing both does not.

For a smooth map, Sard theorem supplies a regular value $y$. Its inverse image is discrete and, by compactness, finite. At each $x\in f^{-1}(y)$ the differential is an isomorphism; let its sign be $+1$ or $-1$ according to whether it preserves or reverses the chosen local orientations. The degree as a sum of local degrees isThe sign is computed in oriented manifold charts. The value is independent of the chosen regular value, even when inverse images appear or disappear: the signed count is the coefficient of $[N]$ in $f_{\ast }[M]$.

There is a useful local-density expression for the same topological degree. If $\omega$ is a volume form with ${\int }_N\omega =1$, thenThis degree by integration of a pullback volume form follows first by choosing a smooth top-form supported in a small neighborhood of a regular value, where the inverse branches contribute their orientation signs. Any other normalized top-form differs from it by an exact form: integration identifies $H_{\mathrm{dR}}^n(N)$ with $\mathbb{R}$. The integral of its pullback difference vanishes by Stokes theorem. In particular, for any top-form $\eta$, ${\int }_M{f}^{\ast }\eta =\mathrm{deg}(f){\int }_N\eta$.

A homotopy $H:M\times [0,1]\to N$ preserves this integral, since $d\omega =0$ and Stokes theorem givesThus topological degree is a homotopy invariant. It is multiplicative under composition, because the induced maps on homology compose: $\mathrm{deg}(g\circ f)=\mathrm{deg}(g)\mathrm{deg}(f)$. The identity has degree one, a constant map has degree zero for $n>0$, and an orientation-reversing diffeomorphism has degree minus one. An orientation-preserving finite covering map has degree equal to its number of sheets. Nonzero topological degree forces surjectivity, since an omitted point would be a regular value with an empty inverse image.

For the circle, $e^{i\theta }\mapsto e^{ik\theta }$ has degree $k$, positive or negative. This is its winding number, computable as $(2\pi i{)}^{-1}\int f^{-1}df$. The antipodal map of $S^n$ has degree $(-1{)}^{n+1}$: its extension $-I$ on the ambient $(n+1)$-dimensional vector space has that determinant sign and respects the outward-normal convention. A holomorphic map $z\mapsto z^k$, $k\ge 1$, on the Riemann sphere has degree $k$, whereas its complex conjugate has degree $-k$. These examples show how orientation, rather than simply the number of inverse images, determines the integer.

For maps $S^n\to S^n$, topological degree gives the complete homotopy classification ${\pi }_n(S^n)\cong \mathbb{Z}$. The degree does not classify general manifold maps: the identity of the torus and the map induced by the integer matrix $\left(\begin{array}{cc}1& 1\\ 0& 1\end{array}\right)$ both have degree one, but have different induced maps on $H_1(T^2;\mathbb{Z})$ and so are not homotopic. A nonzero-degree map $S^n\to S^n$ cannot extend continuously to $B^{n+1}$, because such an extension would make the boundary map null-homotopic. In the smooth setting, Stokes theorem gives the same obstruction by applying it to the pulled-back normalized volume form.

The hypotheses can be adjusted, but must be stated. For connected oriented noncompact manifolds, a proper map has a degree defined using compactly supported top-forms, and it is invariant under proper homotopies. For manifolds with boundary one uses relative fundamental classes and maps of pairs, or fixes appropriate boundary conditions. Without an integral orientation one can still count inverse images modulo two, obtaining a mod-two degree. The integer integral formula used below assumes the oriented setting.

In classical field theory, these ideas turn continuous fields into quantized topological charges. Suppose a field on ${\mathbb{R}}^d$ approaches one fixed target value at spatial infinity. The one-point compactification makes it a map $\varphi :S^d\to \mathcal{V}$. When the target $\mathcal{V}$ is an oriented closed $d$-manifold, its topological degree labels topological sectors. More generally the sectors are described by homotopy groups; an integer degree is available only when the domain and target have the appropriate dimensions and orientations. Smooth time evolution preserving the boundary condition is a homotopy, so it cannot change the integer. A change requires a singular field, escape from the allowed target, or a change at the boundary.

A normalized closed target $d$-form gives the pullback-volume representation of a topological current. On spacetime, put $\alpha ={\varphi }^{\ast }\omega$. Since $d\alpha ={\varphi }^{\ast }(d\omega )=0$, its dual current is identically conserved, andis independent of time when there is no flux at infinity. This conservation law follows from geometry without using the field equations; it need not arise from a continuous symmetry through Noether theorem.

A concrete example is the O3 nonlinear sigma model in two spatial dimensions. Its unit-vector field $\mathbf{n}$ approaches a constant at infinity, defining $S^2\to S^2$. The normalized area form of the target gives the degree charge of an O3 sigma-model lump:For the energy normalization $E=\frac{1}{2}\int (|{\partial }_1\mathbf{n}{|}^2+|{\partial }_2\mathbf{n}{|}^2)$, the identities $\mathbf{n}\cdot {\partial }_i\mathbf{n}=0$ giveThis is the Bogomolny degree bound for the O3 sigma model. Choosing the sign appropriate to $Q$ makes the square nonnegative; vanishing of the square gives first-order Bogomolny equations and a sigma-model lump saturating the bound. With the oriented stereographic projectionthe maps $w=z^k$ have $Q=k$ and $E=4\pi k$. Their conjugates have $Q=-k$ with the same energy. Holomorphic rational maps have positive degree equal to their degree as rational maps; taking a reciprocal does not reverse the orientation. Antiholomorphic dependence reverses it.

The Skyrme model supplies a three-dimensional example. A field $U:{\mathbb{R}}^3\to \mathrm{SU}(2)$ with $U\to I$ at infinity is a map $S^3\to \mathrm{SU}(2)\cong S^3$. Take $T_i=-i{\tau }_i$ and $U^{-1}dU={\theta }^i{T}_i$, with ${\theta }^1\wedge {\theta }^2\wedge {\theta }^3$ positive. Since $\mathrm{tr}(T_i{T}_j{T}_k)=-2{ϵ}_{ijk}$, the normalized target volume form isThe integral is one on the unit three-sphere. Consequently the Skyrme baryon number as a mapping degree isThis is the topological baryon number in the Skyrme model; the sign has been fixed by the stated orientation and anti-Hermitian generator convention.

A topological charge alone does not guarantee a stable finite-size solution. The degree and energetic stability of a field configuration concern different properties. For a three-dimensional configuration of size $R$, the two-derivative energy scales as $R$, so it can decrease by shrinking while the topological degree remains fixed for every $R>0$. The limit can be singular. The Skyrme term, with four derivatives, scales as $R^{-1}$ and can balance the shrinking tendency. This is the role of Derrick theorem in distinguishing topological obstruction from energetic stability.

For defects, the relevant boundary map can instead be the sphere surrounding a core. A vacuum manifold equal to $S^1$ gives the integer winding number of a vortex; a vacuum manifold $S^2$ gives the degree of a surrounding $S^2$ for a magnetic monopole. This vacuum-boundary degree as a defect charge obstructs extending the normalized vacuum field through the enclosed ball. A nonzero integer therefore forces the field to leave the vacuum manifold somewhere in the core. This construction does not require the field to take one constant value in every direction at infinity.

Degree also appears in four-dimensional gauge theory through a boundary transition function. For an anti-Hermitian SU(2) gauge connection on ${\mathbb{R}}^4$, write $F=dA+A\wedge A$ and assume finite-action boundary behavior $A\to g^{-1}dg$ on the large bounding three-sphere. In the second-Chern conventionthe identity $d\mathrm{tr}(A\wedge dA+\frac{2}{3}{A}^3)=\mathrm{tr}(F\wedge F)$ and the Maurer-Cartan equation giveThis boundary winding representation of Yang-Mills topological charge relates the Second Chern number to the degree of $g:S^3\to \mathrm{SU}(2)$. The Chern-Simons 3-form turns the bulk integral into the boundary winding integral. Conventions which define the instanton number with the opposite trace sign reverse $k$; the integer quantization is unchanged. A Yang-Mills theta term weights a sector by $e^{i\vartheta k}$, giving periodicity $\vartheta \mapsto \vartheta +2\pi$. Thus the same topological degree that counts oriented inverse images also labels field sectors and expresses their quantized charges as integrals of local densities.

A Bogomolny bound expresses a static field energy as nonnegative squares plus a term fixed by the topological charge or boundary data. Setting the squares to zero gives the first-order Bogomolny equations. Their solutions minimize the energy in that sector and satisfy the second-order Euler-Lagrange field equations, although a general stationary solution need not attain the bound.

For one real scalar in one spatial dimension, take the Lagrangian densityA finite-energy field configuration in this static sector must approach scalar-field vacua ${\varphi }_{\pm }$ with $W^{\prime }({\varphi }_{\pm })=0$ at the two ends. Completing the square gives the square completion for a one-dimensional kink:Choose the sign for which the boundary term is nonnegative. ThusTaking the derivative of the equality equation gives ${\varphi }_{xx}=W^{\prime }{W}^{\prime \prime }=d([W^{\prime }{]}^2/2)/d\varphi$, the static Euler-Lagrange field equation. The boundary term is invariant under deformations keeping the asymptotic scalar-field vacua fixed; it is not a contribution from the local shape of the kink.

For a phi-four kink, let $c>0$, $W(\varphi )=c^2\varphi -{\varphi }^3/3$, and select the sector ${\varphi }_{-}=-c$, ${\varphi }_{+}=c$. Then $W^{\prime }=c^2-{\varphi }^2$, and the increasing Bogomolny equation is ${\varphi }_x=c^2-{\varphi }^2$. Separating variables yieldsThe integration constant $X$ is the translational collective coordinate. Directly, ${\varphi }_K^{\prime }=c^2{\mathrm{sech}}^2(c(x-X))$ and its energy density is $c^4{\mathrm{sech}}^4(c(x-X))$, whose integral is $4c^3/3$. The decreasing antikink has the reversed boundary values, profile $-{\varphi }_K$, and the same energy. A Lorentz boost produces the exact uniformly moving kink $c\tanh [c\gamma (x-X_0-vt)]$, with Lorentz factor $\gamma =(1-v^2{)}^{-1/2}$ and energy $\gamma M$.

In two spatial dimensions, choose the Abelian Higgs model at critical coupling, with $D_i={\partial }_i-i{A}_i$, $B={\partial }_1{A}_2-{\partial }_2{A}_1$, and energyThe gauge covariant derivative is used throughout this normalization. Integration by parts, using $[D_1,D_2]=-iB$, gives $\int |D_i\varphi {|}^2=\int |(D_1+i{D}_2)\varphi {|}^2+\int B|\varphi {|}^2$, with a vanishing boundary divergence for the decaying vortex fields. Combining this identity with the magnetic and potential terms gives the Bogomolny square completion for an Abelian Higgs vortex:Finite-energy field configurations have $|\varphi |\to 1$ at infinity, and $D_i\varphi \to 0$ makes the magnetic flux equal to the phase winding $2\pi N$. For $N>0$,These are the Bogomolny vortex equations for an Abelian Higgs vortex. Opposite signs give antivortices and the bound $E\ge \pi |N|$. The coefficient $\pi$ follows from the explicit energy normalization above; other conventions can give $2\pi$. Static solutions of fixed positive vortex number have equal energy independent of their positions, giving the Abelian Higgs vortex moduli space used for slow dynamics.

For a Bogomolny-Prasad-Sommerfield monopole, the dimensionless radial Bogomolny equations are $dK/d\rho =-KH$ and $dH/d\rho =(1-K^2)/{\rho }^2$. The displayed profiles solve both equations: differentiating $\rho /\sinh \rho$ gives $K(1/\rho -\coth \rho )=-KH$, while differentiating $H$ gives $1/{\rho }^2-{\mathrm{csch}}^2\rho =(1-K^2)/{\rho }^2$. At the origin $K=1-{\rho }^2/6+O({\rho }^4)$ and $H=\rho /3+O({\rho }^3)$, giving a regular core. At infinity $K$ decays exponentially and $H=1-1/\rho +O(e^{-2\rho })$, giving the vacuum Higgs magnitude and a long-range Abelian magnetic field.