arXiv subjects

Changhyun Ahn

Publications and source records attributed to Changhyun Ahn.

At least 37 records · Page 2Linked to original sources

arXiv2019

The ${\cal N}=4$ Coset Model and the Higher Spin Algebra

By computing the operator product expansions between the first two ${\cal N}=4$ higher spin multiplets in the unitary coset model, the (anti)commutators of higher spin currents are obtained under the large $(N,k)$ 't Hooft-like limit. The free field realization with complex bosons and fermions is presented. The (anti)commutators for generic spins $s_1$ and $s_2$ with manifest $SO(4)$ symmetry at vanishing 't Hooft-like coupling constant are completely determined. The structure constants can be written in terms of the ones in the ${\cal N}=2$ ${\cal W}_{\infty}$ algebra found by Bergshoeff, Pope, Romans, Sezgin and Shen previously, in addition to the spin-dependent fractional coefficients and two $SO(4)$ invariant tensors. We also describe the ${\cal N}=4$ higher spin generators, by using the above coset construction results, for general super spin $s$ in terms of oscillators in the matrix generalization of $AdS_3$ Vasiliev higher spin theory at nonzero 't Hooft-like coupling constant. We obtain the ${\cal N}=4$ higher spin algebra for low spins and present how to determine the structure constants, which depend on the higher spin algebra parameter, in general, for fixed spins $s_1$ and $s_2$.

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arXiv2019

A Supersymmetric Enhancement of ${\cal N}=1$ Holographic Minimal Model

By studying the ${\cal N}=1$ holographic minimal model at the "critical" level, we obtain the lowest ${\cal N}=2$ higher spin multiplet of spins $(\frac{3}{2}, 2, 2, \frac{5}{2})$ in terms of two adjoint fermion types for generic $N$. We subsequently determine operator product expansions between the lowest and second lowest (${\cal N}=2$) higher spin multiplet of spins $(3, \frac{7}{2}, \frac{7}{2}, 4)$, and the corresponding Vasiliev's oscillator formalism with matrix generalization on $AdS_3$ higher spin theory in the extension of $OSp(2|2)$ superconformal algebra. Under the large $N$ limit (equivalent to large central charge) in the extension of ${\cal N}=2$ superconformal algebra in two dimensions, operator product expansions provide asymptotic symmetry algebra in $AdS_3$ higher spin theory.

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arXiv2019

The Operator Product Expansions in the ${\cal N}=4$ Orthogonal Wolf Space Coset Model

Some of the operator product expansions (OPEs) between the lowest $SO(4)$ singlet higher spin-$2$ multiplet of spins $(2, \frac{5}{2}, \frac{5}{2}, \frac{5}{2}, \frac{5}{2}, 3, 3, 3, 3, 3, 3, \frac{7}{2}, \frac{7}{2}, \frac{7}{2}, \frac{7}{2}, 4)$ in an extension of the large ${\cal N}=4$ (non)linear superconformal algebra were constructed in the ${\cal N}=4$ superconformal coset $\frac{SO(N+4)}{SO(N) \times SO(4)}$ theory with $N=4$ previously. In this paper, by rewriting the above OPEs with $N=5$, the remaining undetermined OPEs are completely determined. There exist additional $SO(4)$ singlet higher spin-$2$ multiplet, six $SO(4)$ adjoint higher spin-$3$ multiplets, four $SO(4)$ vector higher spin-$\frac{7}{2}$ multiplets, $SO(4)$ singlet higher spin-$4$ multiplet and four $SO(4)$ vector higher spin-$\frac{9}{2}$ multiplets in the right hand side of these OPEs. Furthermore, by introducing the arbitrary coefficients in front of the composite fields in the right hand sides of the above complete 136 OPEs, the complete structures of the above OPEs are obtained by using various Jacobi identities for generic $N$. Finally, we describe them as one single ${\cal N}=4$ super OPE between the above lowest $SO(4)$ singlet higher spin-$2$ multiplet in the ${\cal N}=4$ superspace.

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arXiv2018

Chiral Algebras of Two-Dimensional SYK Models

We study chiral algebras in the $\bar{Q}$-cohomology of two dimensional SYK models with extended supersymmetry. In a special limit discovered in arXiv:1805.09325, we are able to construct explicitly a "vertical" single-particle higher-spin algebra that is bilinear in the fundamental fields. This algebra can be regarded as the counterpart, when going away from criticality, of the infrared emergent higher-spin symmetry of the $\mathcal{N}=(0,2)$ SYK model. Moreover, a second "horizontal" single-particle higher-spin algebra appears in this limit. Together with the vertical algebra they generate a stringy algebra with a "higher spin square" structure that is believed to appear in the tensionless limit of string theory. On the other hand, we do not find single-particle higher-spin algebra away from the special limit, which is consistent with the result in arXiv:1805.09325. Our analysis is carried out for each individual realization of the random couplings and for finite $N$ (and $M$), which in particular indicates that the conclusion in arXiv:1805.09325 is robust to $1/N$ corrections.

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arXiv2018

Higher Spin Currents with Manifest $SO(4)$ Symmetry in the Large ${\cal N}=4$ Holography

The large ${\cal N}=4$ nonlinear superconformal algebra is generated by six spin-$1$ currents, four spin-$\frac{3}{2}$ currents and one spin-$2$ current. The simplest extension of these $11$ currents is described by the $16$ higher spin currents of spins $(1,\frac{3}{2},\frac{3}{2}, \frac{3}{2}, \frac{3}{2}, 2,2,2,2,2,2, \frac{5}{2}, \frac{5}{2},\frac{5}{2}, \frac{5}{2}, 3)$. In this paper, by using the defining operator product expansions (OPEs) between the $11$ currents and $16$ higher spin currents, we determine the $16$ higher spin currents (the higher spin-$1, \frac{3}{2}$ currents were found previously) in terms of affine Kac-Moody spin-$\frac{1}{2}, 1$ currents in the Wolf space coset model completely. An antisymmetric second rank tensor, three antisymmetric almost complex structures or the structure constant are contracted with the multiple product of spin-$\frac{1}{2}, 1$ currents. The Wolf space coset contains the group $SU(N+2)$ and the level $k$ is characterized by the affine Kac-Moody spin-$1$ currents. After calculating the eigenvalues of the zeromode of the higher spin-$3$ current acting on the higher representations up to three (or four) boxes of Young tableaux in $SU(N+2)$ in the Wolf space coset, we obtain the corresponding three-point functions with two scalar operators at finite $(N,k)$. Furthermore, under the large $(N,k)$ 't Hooft like limit, the eigenvalues associated with any boxes of Young tableaux are obtained and the corresponding three-point functions are written in terms of the 't Hooft coupling constant in simple form in addition to the two-point functions of scalars and the number of boxes.

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arXiv2017

Wolf Space Coset Spectrum in the Large ${\cal N}=4$ Holography

After reviewing the four eigenvalues (the conformal dimension, two $SU(2)$ quantum number, and $U(1)$ charge) in the minimal (and higher) representations in the Wolf space coset where the ${\cal N}=4$ superconformal algebra is realized by $11$ currents in nonlinear way, these four eigenvalues in the higher representations up to three boxes (of Young tableaux) are examined in detail. The eigenvalues associated with the higher spin-$1,2,3$ currents in the (minimal and) higher representations up to two boxes are studied. They are expressed in terms of the two finite parameters $(N, k)$ where the Wolf space coset contains the group $SU(N+2)$ and the affine Kac-Moody spin $1$ current has the level $k$. Under the large $(N,k)$ 't Hooft-like limit, they are simply linear combinations of the eigenvalues in the minimal representations. For the linear case where the ${\cal N}=4$ superconformal algebra is realized by $16$ currents in linear way, the eigenvalues, corresponding to the spin $2$ current and the higher spin $3$ current, which are the only different quantities (compared to the nonlinear case), are also obtained at finite $(N,k)$. They coincide with the results for the nonlinear case above after the large $(N,k)$ 't Hooft-like limit is taken. As a by product, the three-point functions of the higher spin currents with two scalar operators can be determined at finite $(N,k)$.

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arXiv2017

The Next $16$ Higher Spin Currents and Three-Point Functions in the Large ${\cal N}=4$ Holography

By using the known operator product expansions (OPEs) between the lowest $16$ higher spin currents of spins $(1, \frac{3}{2}, \frac{3}{2}, \frac{3}{2}, \frac{3}{2}, 2,2,2,2,2,2, \frac{5}{2}, \frac{5}{2}, \frac{5}{2}, \frac{5}{2}, 3)$ in an extension of the large ${\cal N}=4$ linear superconformal algebra, one determines the OPEs between the lowest $16$ higher spin currents in an extension of the large ${\cal N}=4$ nonlinear superconformal algebra for generic $N$ and $k$. The Wolf space coset contains the group $G =SU(N+2)$ and the affine Kac-Moody spin $1$ current has the level $k$. The next $16$ higher spin currents of spins $(2,\frac{5}{2}, \frac{5}{2}, \frac{5}{2}, \frac{5}{2}, 3,3,3,3,3,3, \frac{7}{2}, \frac{7}{2}, \frac{7}{2}, \frac{7}{2},4)$ arise in the above OPEs. The most general lowest higher spin $2$ current in this multiplet can be determined in terms of affine Kac-Moody spin $\frac{1}{2}, 1$ currents. By careful analysis of the zero mode (higher spin) eigenvalue equations, the three-point functions of bosonic higher spin $2, 3, 4$ currents with two scalars are obtained for finite $N$ and $k$. Furthermore, we also analyze the three-point functions of bosonic higher spin $2, 3, 4$ currents in the extension of the large ${\cal N}=4$ linear superconformal algebra. It turns out that the three-point functions of higher spin $2,3$ currents in the two cases are equal to each other at finite $N$ and $k$. Under the large $(N,k)$ 't Hooft limit, the two descriptions for the three-point functions of higher spin $4$ current coincide with each other. The higher spin extension of $SO(4)$ Knizhnik Bershadsky algebra is described.

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arXiv2017

Higher Spin Currents in the Orthogonal Coset Theory

In the coset model $(D_N^{(1)} \oplus D_N^{(1)},D_N^{(1)})$ at levels $(k_1,k_2)$, the higher spin $4$ current that contains the quartic WZW currents contracted with completely symmetric $SO(2N)$ invariant $d$ tensor of rank $4$ is obtained. The three-point functions with two scalars are obtained for any finite $N$ and $k_2$ with $k_1=1$. They are determined also in the large $N$ 't Hooft limit. When one of the levels is the dual Coxeter number of $SO(2N)$, $k_1=2N-2$, the higher spin $\frac{7}{2}$ current, which contains the septic adjoint fermions contracted with the above $d$ tensor and the triple product of structure constants, is obtained from the operator product expansion (OPE) between the spin $\frac{3}{2}$ current living in the ${\cal N}=1$ superconformal algebra and the above higher spin $4$ current. The OPEs between the higher spin $\frac{7}{2}, 4$ currents are described. For $k_1=k_2=2N-2$ where both levels are equal to the dual Coxeter number of $SO(2N)$, the higher spin $3$ current of $U(1)$ charge $\frac{4}{3}$, which contains the six product of spin $\frac{1}{2}$ (two) adjoint fermions contracted with the product of $d$ tensor and two structure constants, is obtained. The corresponding ${\cal N}=2$ higher spin multiplet is determined by calculating the remaining higher spin $\frac{7}{2}, \frac{7}{2}, 4$ currents with the help of two spin $\frac{3}{2}$ currents in the ${\cal N}=2$ superconformal algebra. The other ${\cal N}=2$ higher spin multiplet, whose $U(1)$ charge is opposite to the one of above ${\cal N}=2$ higher spin multiplet, is obtained. The OPE between these two ${\cal N}=2$ higher spin mutiplets is also discussed.

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arXiv2016

Higher Spin Currents in the Enhanced N=3 Kazama-Suzuki Model

The N=3 Kazama-Suzuki model at the `critical' level has been found by Creutzig, Hikida and Ronne. We construct the lowest higher spin currents of spins (3/2, 2,2,2,5/2, 5/2, 5/2, 3) in terms of various fermions. In order to obtain the operator product expansions (OPEs) between these higher spin currents, we describe three N=2 OPEs between the two N=2 higher spin currents denoted by (3/2, 2, 2, 5/2) and (2, 5/2, 5/2, 3) (corresponding 36 OPEs in the component approach). Using the various Jacobi identities, the coefficient functions appearing on the right hand side of these N=2 OPEs are determined in terms of central charge completely. Then we describe them as one single N=3 OPE in the N=3 superspace. The right hand side of this N=3 OPE contains the SO(3)-singlet N=3 higher spin multiplet of spins (2, 5/2, 5/2, 5/2, 3,3,3, 7/2), the SO(3)-singlet N=3 higher spin multiplet of spins (5/2, 3,3,3, 7/2, 7/2, 7/2, 4), and the SO(3)-triplet N=3 higher spin multiplets where each multiplet has the spins (3, 7/2, 7/2, 7/2, 4,4,4, 9/2), in addition to N=3 superconformal family of the identity operator. Finally, by factoring out the spin-1/2 current of N=3 linear superconformal algebra generated by eight currents of spins (1/2, 1,1,1, 3/2, 3/2, 3/2, 2), we obtain the extension of so-called SO(3) nonlinear Knizhnik Bershadsky algebra.

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arXiv2016

Higher Spin Currents in the N=2 Stringy Coset Minimal Model

In the coset model based on (A_{N-1}^{(1)} \oplus A_{N-1}^{(1)}, A_{N-1}^{(1)}) at level (N, N; 2N), it is known that the N=2 superconformal algebra can be realized by the two kinds of adjoint fermions. Each Kac-Moody current of spin-1 is given by the product of fermions with structure constant (f symbols) as usual. One can construct the spin-1 current by combining the above two fermions with the structure constant and the spin-1 current by multiplying these two fermions with completely symmetric SU(N) invariant tensor of rank 3 (d symbols). The lowest higher spin-2 current with nonzero U(1) charge (corresponding to the zeromode eigenvalue of spin-1 current of N=2 superconformal algebra) can be obtained from these four spin-1 currents in quadratic form. Similarly, the other type of lowest higher spin-2 current, whose U(1) charge is opposite to the above one, can be obtained also. Four higher spin-5/2 currents can be constructed from the operator product expansions (OPEs) between the spin-3/2 currents of N=2 superconformal algebra and the above two higher spin-2 currents. The two higher spin-3 currents can be determined by the OPEs between the above spin-3/2 currents and the higher spin-5/2 currents. Finally, the ten N=2 OPEs between the four N=2 higher spin multiplets (2, 5/2, 5/2, 3), (2, 5/2, 5/2, 3), (7/2, 4, 4, 9/2) and (7/2, 4, 4, 9/2) are obtained explicitly for generic N.

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arXiv2015

Three Point Functions in the N=4 Orthogonal Coset Theory

We construct the lowest higher spin-2 current in terms of the spin-1 and the spin-1/2 currents living in the orthogonal SO(N+4)/[SO(N) x SO(4)] Wolf space coset theory for general N. The remaining fifteen higher spin currents are determined. We obtain the three-point functions of bosonic (higher) spin currents with two scalars for finite N and k (the level of the spin-1 current). By multiplying SU(2) x U(1) into the above Wolf space coset theory, the other fifteen higher spin currents together with the above lowest higher spin-2 current are realized in the extension of the large N=4 linear superconformal algebra. Similarly, the three-point functions of bosonic (higher) spin currents with two scalars for finite N and k are obtained. Under the large N 't Hooft limit, the two types of three-point functions in the nonlinear and linear versions coincide as in the unitary coset theory found previously.

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arXiv2015

The Operator Product Expansion between the 16 Lowest Higher Spin Currents in the N=4 Superspace

Some of the operator product expansions (OPEs) between the lowest 16 higher spin currents of spins (1, 3/2, 3/2, 3/2, 3/2, 2, 2, 2, 2, 2, 2, 5/2, 5/2, 5/2, 5/2, 3) in an extension of the large N=4 linear superconformal algebra were constructed in the N=4 superconformal coset SU(5)/SU(3) theory previously. In this paper, by rewriting the above OPEs in the N=4 superspace developed by Schoutens (and other groups), the remaining undetermined OPEs where the corresponding singular terms possess the composite fields with spins s =7/2, 4, 9/2, 5 are completely determined. Furthermore, by introducing the arbitrary coefficients in front of the composite fields in the right hand sides of the above complete 136 OPEs, reexpressing them in the N=2 superspace and using the N=2 OPEs mathematica package by Krivonos and Thielemans, the complete structures of the above OPEs with fixed coefficient functions are obtained with the help of various Jacobi identities. Then one obtains ten N=2 super OPEs between the four N=2 higher spin currents denoted by (1, 3/2, 3/2, 2), (3/2, 2, 2, 5/2), (3/2, 2, 2, 5/2) and (2, 5/2, 5/2, 3) (corresponding 136 OPEs in the component approach) in the N=4 superconformal coset SU(N+2)/SU(N) theory. Finally, one describes them as one single N=4 super OPE between the above sixteen higher spin currents in the N=4 superspace. The fusion rule for this OPE contains the next 16 higher spin currents of spins of (2, 5/2, 5/2, 5/2, 5/2, 3, 3, 3, 3, 3, 3, 7/2, 7/2, 7/2, 7/2, 4) in addition to the quadratic N=4 lowest higher spin multiplet and the large N=4 linear superconformal family of the identity operator. The various structure constants (fixed coefficient functions) appearing in the right hand side of this OPE depend on N and the level k of the bosonic spin-1 affine Kac-Moody current.

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arXiv2015

Three Point Functions in the Large N=4 Holography

The 16 higher spin currents with spins (1, 3/2, 3/2, 2), (3/2, 2, 2, 5/2), (3/2, 2, 2, 5/2) and (2, 5/2, 5/2, 3) in an extension of large N=4 `nonlinear' superconformal algebra in two dimensions were obtained previously. By analyzing the zero mode eigenvalue equations carefully, the three point functions of bosonic (higher spin) currents with two scalars for any finite N (where SU(N+2) is the group of coset) and k (the level of spin-1 Kac Moody current) are obtained. Furthermore, the 16 higher spin currents with above spin contents in an extension of large N=4 `linear' superconformal algebra are obtained for generic N and k implicitly. The corresponding three point functions are also determined. Under the large N 't Hooft limit, the two corresponding three point functions in the nonlinear and linear versions coincide with each other although they are completely different for finite N and k.

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arXiv2015

Higher Spin Currents in Wolf Space: Part III

The large N=4 linear superconformal algebra (generated by four spin-1/2 currents, seven spin-1 currents, four spin-3/2 currents and one spin-2 current) found by Sevrin, Troost and Van Proeyen (and other groups) was realized in the N=4 superconformal coset SU(5)/SU(3) theory previously. The lowest 16 higher spin currents of spins (1, 3/2, 3/2, 2), (3/2, 2, 2, 5/2), (3/2, 2, 2, 5/2) and (2, 5/2, 5/2, 3) are obtained by starting with the operator product expansions (OPEs) between the four spin-3/2 currents from the above large N=4 linear superconformal algebra and the lowest higher spin-1 current which is the same as the one in the Wolf space coset SU(5)/[SU(3) x SU(2) x U(1)] theory. These OPEs determine the four higher spin-3/2 currents and the next six higher spin-2 currents are obtained from the OPEs between the above four spin-3/2 currents associated with the N=4 supersymmetry and these four higher spin-3/2 currents. The four higher spin-5/2 currents can be determined by calculating the OPEs between the above four spin-3/2 currents and the higher spin-2 currents. Similarly, the higher spin-3 current is obtained from the OPEs between the four spin-3/2 currents and the higher spin-5/2 currents. The explicit relations between the above 16 higher spin currents and the corresponding 16 higher spin currents which were found in the extension of large N=4 nonlinear superconformal algebra previously are given. By examining the OPEs between the 16 currents from the large N=4 linear superconformal algebra and the 16 higher spin currents, the match with the findings of Beccaria, Candu and Gaberdiel is also given. The next 16 higher spin currents of spins (2, 5/2, 5/2, 3), (5/2, 3, 3, 7/2), (5/2, 3, 3, 7/2) and (3, 7/2, 7/2, 4) occur from the OPEs between the above lowest 16 higher spin currents.

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arXiv2014

Higher Spin Currents in Wolf Space for Generic N

We obtain the 16 higher spin currents with spins (1,3/2,3/2,2),(3/2, 2,2, 5/2), (3/2,2,2, 5/2) and (2,5/2,5/2,3) in the N=4 superconformal Wolf space coset SU(N+2)/[SU(N) x SU(2) x U(1)]. The antisymmetric second rank tensor occurs in the quadratic spin-1/2 Kac-Moody currents of the higher spin-1 current. Each higher spin-3/2 current contains the above antisymmetric second rank tensor and three symmetric (and traceless) second rank tensors (i.e. three antisymmetric almost complex structures contracted by the above antisymmetric tensor) in the product of spin-1/2 and spin-1 Kac-Moody currents respectively. Moreover, the remaining higher spin currents of spins 2, 5/2, 3 contain the combinations of the (symmetric) metric, the three almost complex structures, the antisymmetric tensor or the three symmetric tensors in the multiple product of the above Kac-Moody currents as well as the composite currents from the large N=4 nonlinear superconformal algebra.

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arXiv2014

Higher Spin Currents in Orthogonal Wolf Space

For the N=4 superconformal coset theory by [SO(N+4)/SO(N) x SU(2)] x U(1) (that contains an orthogonal Wolf space) with N=4, the N=2 WZW affine current algebra is obtained. The 16 generators (or 11 generators) of the large N=4 linear (or nonlinear) superconformal algebra are described by these WZW affine currents explicitly. Along the line of large N=4 holography, the extra 16 currents with spins (2,5/2,5/2, 3), (5/2, 3, 3, 7/2), (5/2, 3, 3, 7/2), and (3, 7/2, 7/2, 4) are obtained in terms of the WZW affine currents. The lowest spin of this N=4 multiplet is two rather than one which is for an unitary Wolf space. The operator product expansions (OPEs) between the above 11 currents and these extra 16 higher spin currents are found explicitly.

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arXiv2014

Higher Spin Currents in Wolf Space: Part II

The 16 lowest higher spin currents of spins (1, 3/2, 3/2, 2), (3/2, 2, 2, 5/2 ), (3/2, 2, 2, 5/2) and (2, 5/2, 5/2, 3) in terms of N=2 WZW affine currents were obtained in the N=4 superconformal Wolf space coset SU(5)/[SU(3) x SU(2) x U(1)] previously. By calculating the operator product expansions (OPEs) between the above higher spin currents which are contained in an extension of large N=4 nonlinear superconformal algebra, the next 16 higher spin currents of spins (2, 5/2, 5/2, 3), (5/2, 3, 3, 7/2 ), (5/2, 3, 3, 7/2) and (3, 7/2, 7/2, 4) are determined from the right hand sides of these OPEs. Moreover, the composite fields consisting of both the 11 currents in the large N=4 nonlinear superconformal algebra and the above 16 lowest higher spin currents also occur in the right hand sides of these OPEs. The latter appears quadratically (and linearly) in the fusion rules together with large N=4 nonlinear superconformal family of the identity operator.

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arXiv2014

Higher Spin Currents in Wolf Space: Part I

For the N=4 superconformal coset theory described by SU(N+2)/SU(N) (that contains a Wolf space) with N=3, the N=2 WZW affine current algebra with constraints is obtained. The 16 generators of the large N=4 linear superconformal algebra are described by those WZW affine currents explicitly. By factoring out four spin-1/2 currents and the spin-1 current from these 16 generators, the remaining 11 generators (spin-2 current, four spin-3/2 currents, and six spin-1 currents) corresponding to the large N=4 nonlinear superconformal algebra are obtained. Based on the recent work by Gaberdiel and Gopakumar on the large N=4 holography, the extra 16 currents, with spin contents (1, 3/2, 3/2, 2), (3/2, 2, 2, 5/2), (3/2, 2, 2, 5/2), and (2, 5/2, 5/2, 3) described in terms of N=2 multiplets, are obtained and realized by the WZW affine currents. As a first step towards N=4 W algebra (which is NOT known so far), the operator product expansions (OPEs) between the above 11 currents and these extra 16 higher spin currents are found explicitly. It turns out that the composite fields with definite U(1) charges, made of above (11+16) currents (which commute with the Wolf space subgroup SU(N=3) x SU(2) x U(1) currents), occur in the right hand sides of these OPEs.

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