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# M. Visinescu: Hidden Symmetries of the Five-dimensional Sasaki - Einstein Spaces

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Balkan Workshop BW2013
Beyond the Standard Models
25 – 29 April, 2013, Vrnjačka Banja, Serbia

M. Visinescu: Hidden Symmetries of the Five-dimensional Sasaki - Einstein Spaces

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### M. Visinescu: Hidden Symmetries of the Five-dimensional Sasaki - Einstein Spaces

1. 1. Hidden symmetries of theﬁve-dimensional Sasaki-Einstein metricsMihai VisinescuDepartment of Theoretical PhysicsNational Institute for Physics and Nuclear Engineering ”Horia Hulubei”Bucharest, RomaniaBALKAN WORKSHOP 2013– Beyond the Standard Models –Vrnjaˇcka Banja, Serbia, 25 – 29 April, 2013
2. 2. Outline1. Symmetries and conserved quantities2. Killing forms3. K¨ahler, Sasaki manifolds4. Killing forms on Kerr-NUT-(A)dS spaces5. Y(p, q) spaces6. Killing forms on mixed 3-Sasakian manifolds7. Outlook
3. 3. Symmetries and conserved quantities (1)Let (M, g) be a n-dimensional manifold equipped with a(pseudo-)Riemmanian metric g and denote byH =12gijpipj ,the Hamilton function describing the motion in a curved space.In terms of the phase-space variables(xi, pi) the Poissonbracket of two observables P, Q is{P, Q} =∂P∂xi∂Q∂pi−∂P∂pi∂Q∂xi.
4. 4. Symmetries and conserved quantities (2)A conserved quantity of motions expanded as a power series inmomenta:K = K0 +pk=11k!Ki1···ik (x)pi1· · · pik.Vanishing Poisson bracket with the Hamiltonian, {K, H} = 0,impliesK(i1···ik ;i)= 0 ,Such symmetric tensor Ki1···ikk is called a St¨ackel-Killing (SK)tensor of rank k
5. 5. Killing forms (1)A vector ﬁeld X on a (pseudo-)Riemannian manifold (M, g) issaid to be a Killing vector ﬁeld if the Levi-Civita connectionof g satisﬁesg( Y X, Z) + g(Y, Z X) = 0,for all vector ﬁelds Y, Z on M.
6. 6. Killing forms (2)A natural generalization of Killing vector ﬁelds is given by theconformal Killing vector ﬁelds , i.e. vector ﬁelds with a ﬂowpreserving a given conformal class of metrics.More general, a conformal Killing-Yano tensor( also called conformal Yano tensor or conformal Killingform or twistor form ) of rank p on a (pseudo-) Riemannianmanifold (M, g) is a p -form ω which satisﬁes:X ω =1p + 1X−| dω −1n − p + 1X∗∧ d∗ω,for any vector ﬁeld X on M, where n is the dimension of M , X∗is the 1-form dual to the vector ﬁeld X with respect to the metricg , −| is the operator dual to the wedge product and d∗ is theadjoint of the exterior derivative d.
7. 7. Killing forms (3)If ω is co-closed then we obtain the deﬁnition of a Killing-Yanotensor, also called Yano tensor or Killing form :ωi1...ik−1(ik ;j) = 0 .Moreover, a Killing form ω is said to be a special Killing form ifit satisﬁes for some constant c the additional equationX (dω) = cX∗∧ ω ,for any vector ﬁeld X on M .These two generalizations of the Killing vectors could berelated. Given two Killing-Yano tensors ωi1,...,ir and σi1,...,ir it ispossible to associate with them a St¨ackel-Killing tensor of rank2 :K(ω,σ)ij = ωii2...irσ i2...irj + σii2...irω i2...irj .
8. 8. K¨ahler, Sasaki manifolds (1)Even dimensions (1)An almost Hermitian structure on a smooth manifold M is apair (g, J), where g is a Riemannian metric on M and J is analmost complex structure on M, which is compatible with g, i.e.g(JX, JY) = g(X, Y),for all vector ﬁelds X, Y on M. In this case, the triple (M, J, g) iscalled an almost Hermitian manifold.If J is parallel with respect to the Levi-Civita connection of g,then (M, J, g) is said to be a K¨ahler manifold.On a K¨ahler manifold, the associated K¨ahler form, i.e thealternating 2-form Ω deﬁned byΩ(X, Y) = g(JX, Y)is closed.
9. 9. K¨ahler, Sasaki manifolds (2)Even dimensions (2)In local holomorphic coordinates (z1, ..., zm), the associatedK¨ahler form Ω can be written asΩ = igj¯k dzj∧ d¯zk= X∗j ∧ Y∗j =i2Z∗j ∧ ¯Z∗j ,where (X1, Y1, ..., Xm, Ym) is an adapted local orthonormal ﬁeld(i.e. such that Yj = JXj ), and (Zj, ¯Zj) is the associated complexframe given byZj =12(Xj − iYj), ¯Zj =12(Xj + iYj) .
10. 10. K¨ahler, Sasaki manifolds (3)Even dimensions (3)Volume form isdV =1m!Ωm,where dV denotes the volume form of M, Ωm is the wedgeproduct of Ω with itself m times, m being the complex dimensionof M . Hence the volume form is a real (m, m)-form on M.If the volume of a K¨ahler manifold is written asdV = dV ∧ d ¯Vthen dV is the complex volume holomorphic (m, 0) form of M.The holomorphic form of a K¨ahler manifold can be written in asimple way with respect to any (pseudo-)orthonormal basis,using complex vierbeins ei + Jei. Up to a power factor of theimaginary unit i, the complex volume is the exterior product ofthese complex vierbeins.
11. 11. K¨ahler, Sasaki manifolds (4)Odd dimensions (1)Let M be a smooth manifold equipped with a triple (ϕ, ξ, η),where ϕ is a ﬁeld of endomorphisms of the tangent spaces, ξ isa vector ﬁeld and η is a 1-form on M.If we have:ϕ2= −I + η ⊗ ξ , η(ξ) = 1then we say that (ϕ, ξ, η) is an almost contact structure on M.
12. 12. K¨ahler, Sasaki manifolds (5)Odd dimensions (2)A Riemannian metric g on M is said to be compatible with thealmost contact structure (ϕ, ξ, η) iff the relationg(ϕX, ϕY) = g(X, Y) − η(X)η(Y)holds for all pair of vector ﬁelds X, Y on M.An almost contact metric structure (ϕ, ξ, η, g) is a Sasakianstructure iff the Levi-Civita connection of the metric gsatisﬁes( X ϕ)Y = g(X, Y)ξ − η(Y)Xfor all vector ﬁelds X, Y on M
13. 13. K¨ahler, Sasaki manifolds (6)Metric cone (1)A Sasakian structure may also be reinterpreted andcharacterized in terms of the metric cone as follows. The metriccone of a Riemannian manifold (M, g) is the manifoldC(M) = (0, ∞) × Mwith the metric given by¯g = dr2+ r2g,where r is a coordinate on (0, ∞).M is a Sasaki manifold iff its metric cone C(M) is K¨ahler.The cone C(M) is equipped with an integrable complexstructure J and a K¨ahler 2-form Ω, both of which are parallelwith respect to the Levi-Civita connection ¯ of ¯g.
14. 14. K¨ahler, Sasaki manifolds (7)Metric cone (2)M has odd dimension 2n + 1, where n + 1 is the complexdimension of the K¨ahler cone.The Sasakian manifold (M, g) is naturally isometricallyembedded into the cone via the inclusion{r = 1} × M ⊂ C(M)and the K¨ahler structure of the cone (C(M), ¯g) induces analmost contact metric structure (φ, ξ, η, g) on M.
15. 15. K¨ahler, Sasaki manifolds (8)Einstein manifoldsAn Einstein-Sasaki manifold is a Riemannian manifold (M, g)that is both Sasaki and Einstein, i.e.Ricg = λgfor some real constant λ, where Ricg denotes the Ricci tensorof g. Einstein manifolds with λ = 0 are called Ricci-ﬂatmanifolds.An Einstein-K¨ahler manifold is a Riemannian manifold (M, g)that is both K¨ahler and Einstein. The most important subclassof K¨ahler-Einstein manifolds are the Calabi-Yau manifolds(i. e. K¨ahler and Ricci-ﬂat).A Sasaki manifold M is Einstein iff the cone metric C(M) isK¨ahler Ricci-ﬂat. In particular the K¨ahler cone of aSasaki-Einstein manifold has trivial canonical bundle and therestricted holonomy group of the cone is contained in SU(m),where m denotes the complex dimension of the K¨ahler cone.
16. 16. K¨ahler, Sasaki manifolds (9)Progression from Einstein-K¨ahler to Einstein-Sasaki to Calabi-Yau manifolds (1)Suppose we have an Einstein-Sasaki metric gES on a manifoldM2n+1 of odd dimension 2n + 1. An Einstein-Sasaki manifoldcan always be written as a ﬁbration over an Einstein-K¨ahlermanifold M2n with the metric gEK twisted by the overall U(1)part of the connectionds2ES = (dψn + 2A)2+ ds2EK ,where dA is given as the K¨ahler form of the Einstein-K¨ahlerbase. The metric of the cone manifold M2n+2 = C(M2n+1) isds2cone = dr2+ r2ds2ES = dr2+ r2(dψn + 2A)2+ ds2EK .
17. 17. K¨ahler, Sasaki manifolds (10)Progression from Einstein-K¨ahler to Einstein-Sasaki to Calabi-Yau manifolds (2)The cone manifold is Calabi-Yau and its K¨ahler form isΩcone = rdr ∧ (dψn + 2A) + r2ΩEK ,and the K¨ahler condition dΩcone = 0 impliesdA = ΩEK ,where ΩEK is K¨ahler form of the Einstein-K¨ahler base manifoldM2n.The Sasakian 1-form of the Einstein-Sasaki metric isη = 2A + dψn ,which is a special unit-norm Killing 1-form obeying for all vectorﬁelds XX η =12X−| dη , X (dη) = −2X∗∧ η .
18. 18. K¨ahler, Sasaki manifolds (11)Hidden symmetries (1)The hidden symmetries of the Sasaki manifold M2n+1 aredescribed by the special Killing (2k + 1)−formsΨk = η ∧ (dη)k, k = 0, 1, · · · , n.Besides these Killing forms, there are n closed conformalKilling forms ( also called ∗-Killing forms)Φk = (dη)k, k = 1, · · · , n.
19. 19. K¨ahler, Sasaki manifolds (12)Hidden symmetries (2)Moreover, in the case of holonomy SU(n + 1) , i.e. the coneM2n+2 = C(M2n+1) is K¨ahler and Ricci-ﬂat, or equivalentlyM2n+1 is Einstein-Sasaki, it follows that we have two additionalKilling forms of degree n + 1 on the manifold M2n+1. Theseadditional Killing forms are connected with the additionalparallel forms of the Calabi-Yau cone manifold M2n+2 given bythe complex volume form and its conjugate.
20. 20. K¨ahler, Sasaki manifolds (13)Hidden symmetries (3)In order to extract the corresponding additional Killing forms onthe Sasaki-Einstein space we make use of the fact that for anyp -form ω on the space M2n+1 we can deﬁne an associated(p + 1) -form ωC on the cone C(M2n+1)ωC:= rpdr ∧ ω +rp+1p + 1dω .The 1-1-correspondence between special Killing p -forms onM2n+1 and parallel (p + 1) -forms on the metric cone C(M2n+1)allows us to describe the additional Killing forms onSasaki-Einstein spaces.
21. 21. Killing forms on Kerr-NUT-(A)dS spaces (1)In the case of Kerr-NUT-(A)dS spacetime the Einstein-K¨ahlermetric gEK and the K¨ahler potential A aregEK =∆µdx2µXµ+Xµ∆µn−1j=0σ(j)µ dψj2,Xµ = −4n+1i=1(αi − xµ) − 2bµ ,A =n−1k=0σ(k+1)dψk ,
22. 22. Killing forms on Kerr-NUT-(A)dS spaces (2)with∆µ =ν=µ(xν − xµ) ,σ(k)µ =ν1<···<νkνi =µxν1. . . xνk, σ(k)=ν1<···<νkxν1. . . xνk.Here, coordinates xµ (µ = 1, . . . , n) stands for the Wick rotatedradial coordinate and longitudinal angles and the Killingcoordinates ψk (k + 0, . . . , n − 1) denote time and azimuthalangles with Killing vectors ξ(k) = ∂ψk.αi (i = 1, . . . , n + 1) and bµ are constants related to thecosmological constant, angular momenta, mass and NUTparameters.
23. 23. Killing forms on Kerr-NUT-(A)dS spaces (3)Write the metric gEK on the Einstein-K¨ahler manifold M2n in theformgEK = oˆµoˆµ+ ˜oˆµ˜oˆµ,and the K¨ahler 2-formΩ = dA = oˆµ∧ ˜oˆµ.whereoˆµ=∆µXµ(xµ)dxµ ,˜oˆµ=Xµ(xµ)∆µn−1j=0σ(j)µ dψj .
24. 24. Killing forms on Kerr-NUT-(A)dS spaces (4)We introduce the following complex vierbeins on Einstein-K¨ahler manifold M2nζµ = oµ+ i˜oˆµ, µ = 1, · · · , n .On the Calabi-Yau cone manifold M2n+2 the complex vierbeinsare Λµ = rζµ for µ = 1, · · · , n andΛn+1 =drr+ iη .The standard complex volume form of the Calabi-Yau conemanifold M2n+2 isdV = Λ1 ∧ Λ2 ∧ · · · ∧ Λn+1 .
25. 25. Killing forms on Kerr-NUT-(A)dS spaces (5)As real forms we obtain the real part respectively the imaginarypart of the complex volume form. For example, writingΛj = λ2j−1 + iλ2j, j = 1, ..., n + 1,the real part of the complex volume is given byRe dV =[n+12]p=0 1≤i1<i2<...<in+1≤2n+2 (C)(−1)pλi1∧λi2∧...∧λin+1(1)where the condition (C) means that in the second sum aretaken only the indices i1, ..., in+1 such thati1 + ... + in+1 = (n + 1)2 + 2p and (ik , ik+1) = (2j − 1, 2j), for allk ∈ {1, ..., n} and j ∈ {1, ..., n + 1}.
26. 26. Killing forms on Kerr-NUT-(A)dS spaces (6)The imaginary part of the complex volume is given byIm dV =[n2]p=0 1≤i1<i2<...<in+1≤2n+2 (C )(−1)pλi1∧λi2∧...∧λin+1(2)where the condition (C ) in means that in the second sum areconsidered only the indices i1, i2, ..., in+1 such thati1 + ... + in+1 = (n + 1)2 + 2p + 1 and (ik , ik+1) = (2j − 1, 2j), forall k ∈ {1, ..., n} and j ∈ {1, ..., n + 1}.
27. 27. Killing forms on Kerr-NUT-(A)dS spaces (7)Finally, the Einstein-Sasaki manifold M2n+1 is identiﬁed with thesubmanifold {r = 1} of the Calabi-Yau cone manifoldM2n+2 = C(M2n+1) and the additional (n + 1)-Killing forms areaccordingly acquired.The 1-1-correspondence between special Killingp -forms on M2n+1 and parallel (p + 1) -forms on the metriccone C(M2n+1) allows us to describe the additional Killingforms on Einstein-Sasaki spaces.In order to ﬁnd the additional Killing forms on the Einstein -K´ahler manifold M2n+1 we must identify the ωM form in thecomplex volume form of the Calabi-Yau cone.
28. 28. Y(p,q) spaces (1)Inﬁnite family Y(p, q) of Einstein-Sasaki metrics on S2 × S3provides supersymmetric backgrounds relevant to the AdS/CFTcorrespondence. The total space Y(p, q) of an S1-ﬁbration overS2 × S2 with relative prime winding numbers p and q istopologically S2 × S3.Explicit local metric of the 5-dimensional Y(p, q) manifold givenby the line elementds2ES =1 − c y6(dθ2+ sin2θ dφ2) +1w(y)q(y)dy2+q(y)9(dψ − cos θ dφ)2+ w(y) dα +ac − 2y + c y26(a − y2)[dψ − cos θ dφ]2,where
29. 29. Y(p,q) spaces (2)w(y) =2(a − y2)1 − cy, q(y) =a − 3y2 + 2cy3a − y2and a, c are constants. The constant c can be rescaled by adiffeomorphism and in what follows we assume c = 1.The coordinate change α = −16 β − 16 c ψ , ψ = ψ takes the lineelement to the following form ( with p(y) = w(y) q(y) )ds2ES =1 − y6(dθ2+ sin2θ dφ2) +1p(y)dy2+p(y)36(dβ + cos θ dφ)2+19[dψ − cos θ dφ + y(dβ + cos θ dφ)]2,
30. 30. Y(p,q) spaces (3)One can writeds2ES = dS2EK + (13dψ + σ)2The Sasakian 1-form of the Y(p, q) space isη =13dψ + σ ,withσ =13[− cos θ dφ + y(dβ + cos θ dφ)] .connected with local K¨ahler form ΩEK .This form of the metric with the 1-form η is the standard one fora locally Einstein-Sasaki metric with ∂∂ψ the Reeb vector ﬁeld.
31. 31. Y(p,q) spaces (4)The local K¨ahler and holomorphic (2, 0) form for ds2EK areΩEK =1 − y6sin θdθ ∧ dφ +16dy ∧ (dβ + cos θdφ)dVEK =1 − y6p(y)(dθ + i sin θdφ) ∧ dy + ip(y)6(dβ + cos θdφ)
32. 32. Y(p,q) spaces (5)From the isometries SU(2) × U(1) × U(1) the momentaPφ, Pψ, Pα and the Hamiltonian describing the geodesicmotions are conserved. Pφ is the third component of the SU(2)angular momentum, while Pψ and Pα are associated with theU(1) factors. Additionally, the total SU(2) angular momentumgiven byJ2= P2θ +1sin2θ(Pφ + cos θPψ)2+ P2Ψ ,is also conserved.
33. 33. Y(p,q) spaces (6)Speciﬁc conserved quantities for Einstein-Sasaki spaces (1)First of all from the 1-form ηΨ = η ∧ dη=19[(1 − y) sin θ dθ ∧ dφ ∧ dψ + dy ∧ dβ ∧ dψ+ cos θ dy ∧ dφ ∧ dψ − cos θ dy ∧ dβ ∧ dφ+ (1 − y)y sin θ dβ ∧ dθ ∧ dφ] .is a special Killing form. Let us note also thatΨk = (dη)k, k = 1, 2 ,are closed conformal Killing forms ( -Killing forms).
34. 34. Y(p,q) spaces (7)Speciﬁc conserved quantities for Einstein-Sasaki spaces (2)On the Calabi-Yau manifold the K¨ahler form isΩcone = rdr ∧ η + r2ΩEK .and the holomorphic (3, 0) form isdVcone = eψr2dVEK ∧ [dr + ir ∧ η]= eψr2 1 − y6p(y)dθ + i sin θdφ∧ dy + ip(y)6(dβ + cos θdφ)∧ dr + ir3[ydβ + dψ − (1 − y) cos θdφ
35. 35. Y(p,q) spaces (8)Speciﬁc conserved quantities for Einstein-Sasaki spaces (3)The additional Killing 3-forms of the Y(p, q) spaces areextracted from the volume form dVcone.Using the the 1-1-correspondence between special Killing p-forms on M2n+1 and parallel (p + 1) -forms on the metric coneC(M2n+1) for p = 2 we get the following additional Killing2-forms of the Y(p, q) spaces written as real forms:Ξ = Re ωM=1 − y6 p(y)× cos ψ −dy ∧ dθ +p(y)6sin θ dβ ∧ dφ− sin ψ − sin θ dy ∧ dφ −p(y)6dβ ∧ dθ+p(y)6cos θ dθ ∧ dφ
36. 36. Y(p,q) spaces (9)Speciﬁc conserved quantities for Einstein-Sasaki spaces (4)Υ = Im ωM=1 − y6 p(y)× sin ψ −dy ∧ dθ +p(y)6sin θ dβ ∧ dφ+ cos ψ − sin θ dy ∧ dφ −p(y)6dβ ∧ dθ+p(y)6cos θ dθ ∧ dφ
37. 37. Y(p,q) spaces (10)Speciﬁc conserved quantities for Einstein-Sasaki spaces (5)The St¨ackel-Killing tensors associated with the Killing formsΨ , Ξ , Υ are constructed as usual. Together with the Killingvectors Pφ, Pψ, Pα and the total angular momentum J2 theseSt¨ackel-Killing tensors provide the superintegrability of theY(p, q) geometries.
38. 38. Killing forms on mixed 3-Sasakian manifolds (1)3-Sasakian manifolds (1)A Riemannian manifold (M, g) of real dimension m is3-Sasakian if the holonomy group of the metric cone(C(M) , ¯g) = (R+ × M , dr2+ r2g)reduces to a subgroup of Sp m+14 . In particular,m = 4n + 3 , n ≥ 1 and (C(M) , ¯g) is hyperK¨ahler.A 3-Sasakian manifold admits three characteristic vector ﬁelds(ξ1 , ξ2 , ξ3), satisfying any of the corresponding conditions ofthe Sasakian structure, such thatg(ξα , ξβ) = δαβ ,and[ξα , ξβ] = 2 αβγξγ .
39. 39. Killing forms on mixed 3-Sasakian manifolds (2)3-Sasakian manifolds (2)Let (M, g) be a 3-Sasakian manifold and (ϕα, ξα, ηα),α ∈ {1, 2, 3} , be its 3-Sasakian structure. thenηα(ξβ) = δαβ ,ϕα(ξβ) = − αβγξγ ,ϕαϕβ − ξα ⊗ ηβ = − αβγϕγ − δαβI .TheoremEvery 3-Sasakian manifold (M , g) of dimension 4n + 3 isEinstein with Einstein constant λ = 4n + 2.
40. 40. Killing forms on mixed 3-Sasakian manifolds (3)A mixed 3-structure on a smooth manifold M is a triple ofstructures (ϕα, ξα, ηα), α ∈ {1, 2, 3} , which are almostparacontact structures for α = 1, 2 and almost contact structurefor α = 3 , satisfying the following compatibility conditionsηα(ξβ) = 0,ϕα(ξβ) = τβξγ, ϕβ(ξα) = −ταξγ,ηα ◦ ϕβ = −ηβ ◦ ϕα = τγηγ ,ϕαϕβ − ταηβ ⊗ ξα = −ϕβϕα + τβηα ⊗ ξβ = τγϕγ ,where (α, β, γ) is an even permutation of (1, 2, 3) andτ1 = τ2 = −τ3 = −1.
41. 41. Killing forms on mixed 3-Sasakian manifolds (4)If a manifold M with a mixed 3-structure (ϕα, ξα, ηα)α=1,3 admitsa semi-Riemannian metric g such that:g(ϕαX, ϕαY) = τα[g(X, Y) − εαηα(X)ηα(Y)],for all X, Y ∈ Γ(TM) and α = 1, 2, 3 , whereεα = g(ξα, ξα) = ±1then we say that M has a metric mixed 3-structure and g iscalled a compatible metric.
42. 42. Killing forms on mixed 3-Sasakian manifolds (5)A manifold M endowed with a (positive/negative) metric mixed3-structure (ϕα, ξα, ηα, g) is said to be a (positive/negative)mixed 3-Sasakian structure if (ϕ3, ξ3, η3, g) is a Sasakianstructure, while both structures (ϕ1, ξ1, η1, g) and (ϕ2, ξ2, η2, g)are para-Sasakian, i.e.( X ϕα)Y = τα[g(X, Y)ξα − αηα(Y)X]for all vector ﬁelds X, Y on M and α = 1, 2, 3.TheoremAny (4n + 3)−dimensional manifold endowed with a mixed3-Sasakian structure is an Einstein space with Einsteinconstant λ = (4n + 2)θ, with θ = 1, according as the metricmixed 3-structure is positive or negative, respectively.
43. 43. Killing forms on mixed 3-Sasakian manifolds (6)In (mixed) 3-Sasakian case any linear combination of the formsΨk1,k2,k3deﬁned byΨk1,k2,k3=k1k1 + k2 + k3[η1 ∧ (dη1)k1−1] ∧ (dη2)k2∧ (dη3)k3+k2k1 + k2 + k3(dη1)k1∧ [η2 ∧ (dη2)k2−1] ∧ (dη3)k3+k3k1 + k2 + k3(dη1)k1∧ (dη2)k2∧ [η3 ∧ (dη3)k3−1]for arbitrary positive integers k1, k2, k3 , is a special Killing formon M.
44. 44. OutlookComplete integrability of geodesic equationsSeparability of Hamilton-Jacobi, Klein-Gordon, DiracequationsHidden symmetries of other spacetime structures
45. 45. ReferencesM. Visinescu, Mod. Phys. Lett. A 27, 1250217 (2012)M. Visinescu, G. E. Vˆılcu, SIGMA 8, 058 (2012)M. Visinescu, Mod. Phys. Lett. A 26, 2719 (2011)M. Visinescu, SIGMA 7, 037 (2011)M. Visinescu, J. Phys.: Conf. Series 411, 012030 (2013)