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QUASILINEAR EQUATIONS WITH SOURCE TERMS ON CARNOT GROUPS 2 ∗ ∗∗ NGUYEN CONGPHUC ANDIGOR E. VERBITSKY 1 0 2 Abstract. Inthispaperwegivenecessaryandsufficientconditionsfor n the existence of solutions to quasilinear equations of Lane–Emden type a withmeasuredataonaCarnotgroupGofarbitrarystep. Thequasilin- J ear part involves operators of the p-Laplacian type ∆G,p, 1 < p < ∞. 7 These results are based on new a priori estimates of solutions in terms 1 of nonlinear potentials of Th. Wolff’stype. Asaconsequence,we char- acterize completely removable singularities, and prove a Liouville type ] P theorem for supersolutions of quasilinear equations with source terms A which has been known only for equations involving the sub-Laplacian (p=2) on theHeisenberg group. . h t a m [ 1. Introduction 1 v In this paper we study the solvability problem and pointwise estimates of 6 solutions fora class of quasilinear Lane-Emdentypeequations withmeasure 8 data on Carnot groups of arbitrary step. A complete characterization of 5 3 removablesingularitiesforthecorrespondinghomogeneousequationsaswell . as a Liouville type theorem for supersolutions will also be obtained as a 1 0 consequence. 2 The basic setting of our study is a given Carnot group G of step r ≥ 1, 1 i.e., a connected and simply connected stratified nilpotent Lie group whose : v Lie algebra G admits a stratification G = V ⊕V ⊕···⊕V and is generated i 1 2 r X via commutations by its first (horizontal) layer V (see Sect. 2). Given a 1 ar basic {Xj}mj=1 of V1, the associated p-Laplacian operator ∆G,p, 1< p < ∞, is defined by m ∆G,pu= Xi(|Xu|p−2Xiu), i=1 X where Xu = X uX +X uX +···+X uX , 1 1 2 2 m m is the horizontal gradient of u with length |Xu| = m |X u|2 1/2. i=1 i (cid:0)P (cid:1) 2010 Mathematics Subject Classification: Primary 35H20, Secondary 35A01, 20F18. ∗Supported in part by NSFGrant DMS-0901083. ∗∗ Supported in part byNSFGrant DMS-0901550. 1 2 NGUYENCONGPHUCANDIGORE.VERBITSKY We study the following Lane-Emden type equation on a bounded open set Ω⊂ G: (1.1) −∆G,pu = uq +ω in Ω, u = 0 on ∂Ω, (cid:26) where q > p−1> 0, and ω is a given nonnegative finite measure on Ω. Our objective is to obtain necessary and sufficient conditions on the measure ω fortheexistenceofsolutionsto(1.1),andtogiveacompletecharacterization of removable singularities for the corresponding homogeneous equation: (1.2) −∆G,pu= uq in Ω. Equations similar to (1.1) in the entire group G are also considered with applications to Liouville type theorems for the differential inequality (1.3) −∆G,pu ≥uq in G. Such problems have been studied in depth in our previous work [PV1], [PV2] in the standard Euclidean setting; see also earlier work in [BP], [AP], [BV1], and [BV2]. However, in the setting of Carnot groups, the failure of the Besicovitch covering lemma (see [SW], [KR]) and the lack of a perfect dyadic grid of cubes cause major difficulties. We observe that even in the setting of the Heisenberg group, the simplest model of a non-commutative Carnotgroup,Liouvilletypetheoremsforthedifferentialinequality (1.3)are known only in the sub-Laplacian case, i.e., p = 2 (see [GL], [BCC], [PVe]). A substantial part of our study of (1.1) is devoted to integral inequalities forbothlinearandnonlinearpotentialoperatorsandtheirdiscreteanalogues over “approximate” dyadic grids of cubes constructed in [SW] and [Chr] in the general setting of homogeneous spaces. For each α > 0, the Bessel potential of a locally integrable function f in this setting is defined by G (f)(x) = G ∗f(x) = G (y−1x)f(y)dy, x ∈ G, α α ˆ α G where G is the Bessel kernel of order α on G given by α 1 ∞ (1.4) G (x) = tα/2−1e−th(x,t)dt. α Γ(α/2) ˆ 0 In (1.4) h(x,t) is the heat kernel associated with the sub-Laplacian ∆G = ∆G,2 whose basic properties can be found in [Fol], [VSC]. We also write G (fdµ)(x) = G ∗(fdµ)(x) = G (y−1x)f(y)dµ(y), x ∈ G, α α ˆ α G for each locally µ-integrable function f. When dealing with solutions on the entire group G and Liouville type theorems we need to use another linear potential, the Riesz potential. For each 0 < α < M and f ∈ L1 (G), it is defined by loc f(y) I (f)(x) = I ∗f(x)= dy, x ∈ G, α α ˆG dcc(x,y)M−α QUASILINEAR EQUATIONS WITH SOURCE TERMS ON CARNOT GROUPS 3 where d is the Carnot-Carathéodory distance on G, and M is the homo- cc geneous dimension of G (see Sect. 2). Associated with the kernel G is the Bessel capacity C (·), s > 1, α α,s defined by (see [AH], Sec. 2.6, in the Euclidean case) C (E) = inf{kfks :G (f)≥ 1 on E, f ∈Ls(G), f ≥ 0} α,s Ls(G) α for each compact set E ⊂ G. Similarly, the Riesz capacity C˙ (·), 0 < α < α,s M, s > 1, is defined, for a compact set E ⊂ G, by C˙ (E) = inf{kfks : I (f)≥ 1 on E, f ∈ Ls(G), f ≥ 0}. α,s Ls(G) α These capacities will play an essential role in our characterizations of the existence of solutions and removable singularities, as well as Liouville type theoremsfortheLane-Emdentypeequation. Wewillalsoneedthefollowing dual definition of these capacities (see [Lu, Theorem 2.10]; [AH, Theorem 2.2.7] in the Euclidean case): s µ(E) (1.5) C (E) = sup , α,s µ∈M+(E),µ6=0 kGα∗µkLs−s1(G)! and similarly, s µ(E) (1.6) C˙ (E) = sup , α,s µ∈M+(E),µ6=0 kIα∗µkLs−s1(G)! where M+(E) denotes the set of all nonnegative measures supported on E. The nonlinear potential we use below is the (truncated) Wolff’s potential WR originally introduced in [HW]. In our setting, for α > 0, p > 1, and α,p 0< R ≤ ∞, it is defined for each nonnegative measure µ on G by R µ(B (x)) 1 dt WR µ(x) = t p−1 , x ∈ G, α,p ˆ tM−αp t 0 h i where B (x) is the Carnot-Carathéodory ball centered at x of radius t (see t Sect. 2). For our purpose we introduce the following notion of solutions for p- Laplace equations with general measure as data. This will serve as an ef- ficient substitution for the notion of renormalized solutions introduced in [DMOP] in the Euclidean setting. Definition 1.1. For a nonnegative finite measure µ on Ω, we say that u is a solution to (1.7) −∆G,pu = µ in Ω, u = 0 on ∂Ω, (cid:26) in the potential theoretic sense if u is p-superharmonic in Ω, min{u,k} ∈ 1,p S (Ω) for every k > 0, u satisfies a pointwise bound 0 2diam(Ω) (1.8) u(x) ≤ AW (x), ∀x∈ Ω, 1,p 4 NGUYENCONGPHUCANDIGORE.VERBITSKY and for every ϕ ∈C∞(Ω) one has 0 |Xu|p−2Xu·Xϕdx = ϕdµ. ˆ ˆ Ω Ω From this definition we see right away that potential theoretic solutions to (1.7) are also distributional solutions. However, the converse is not necessarily true as easily seen by a simple example (see [Kil]). The existence of potential theoretic solutions to (1.7) will be obtained in Corollary 4.2, whereas their uniqueness is unknown even in the Euclidean setting. InDefinition1.1thenotation S1,p(Ω)standsforthecompletionofC∞(Ω) 0 0 under the norm of the horizontal Sobolev space S1,p(Ω) (see Sect. 3), and in (1.8), A is a universal constant independent of x,u,µ, and Ω. For the notion of p-superharmonic functions on Carnot groups see Sect. 3. We are now ready to state the first result of the paper. Theorem 1.2. Let p > 1, q > p−1, and R = diam(Ω). Suppose that ω is a nonnegative finite measure on Ω such that supp(ω) ⋐ Ω. If the equation (1.9) −∆G,pu = uq +ω in Ω, u = 0 on ∂Ω (cid:26) has a nonnegative p-superharmonic distributional solution u ∈ Lq(Ω), then there exists a constant C > 0 such that statements (i)–(v) below hold true. (i) The inequality q q (1.10) ˆ Gp(f)q−p+1dω ≤ C ˆ fq−p+1dx G G q holds for all f ∈Lq−p+1, f ≥ 0. (ii) For every compact set E ⊂ Ω, ω(E) ≤ CCp, q (E). q−p+1 (iii) The inequality q (1.11) [W2R(gdω)(x)]qdx ≤ C gp−1dω ˆ 1,p ˆ G G q holds for all g ∈ Lp−1(dω), g ≥ 0. (iv) The inequality (1.12) [W2Rω (x)]qdx ≤ Cω(B) ˆ 1,p B B holds for all Carnot-Carathéodory balls B ⊂ G. (v) For all x ∈ Ω, (1.13) W2R[(W2Rω)q](x) ≤CW2Rω(x). 1,p 1,p 1,p Conversely, there exists a constant C = C (M,p,q) > 0 such that if any 0 0 one of the statements (i)–(v) holds with C ≤ C then equation (1.9) has a 0 QUASILINEAR EQUATIONS WITH SOURCE TERMS ON CARNOT GROUPS 5 nonnegative potential theoretic solution u∈ Lq(Ω) for any nonnegative finite measure ω. Moreover, u satisfies the following pointwise estimate u≤ κW2Rω. 1,p Our second result is about removable singularities of solutions to homogeneous equations, which is in fact a consequence of Theorem 1.2. Theorem 1.3. Let q > p−1> 0 and let E be a compact subset of Ω. Then any solution u to the problem u is p-superharmonic in Ω\E, q (1.14) u ∈ L (Ω\E), u≥ 0,  loc  −∆G,pu = uq in D′(Ω\E) is also a solution to a similar problem with Ω in place of Ω\E if and only if  Cp, q (E) = 0. q−p+1 The proof of Theorems 1.2 and 1.3 will be given at the end of Sect. 4. In case the bounded domain Ω in Theorem 1.2 is replaced by the whole group G,thenRieszpotentials andthecorrespondingRieszcapacity mustbeused, and we have the following result. Theorem 1.4. Let 1< p < M, q > p−1 and let ω be a nonnegative locally finite measure on G. If the equation (1.15) −∆G,pu = uq +ω in G, infGu = 0 (cid:26) has a nonnegative p-superharmonic distributional solution u ∈ Lq (G), then loc there exists a constant C > 0 such that statements (i)–(vi) below hold true. (i) For every compact set E ⊂ G, Êuqdx ≤ CC˙p,q−qp+1(E). (ii) The inequality q q ˆ Ip(f)q−p+1dω ≤ C ˆ fq−p+1dx G G q holds for all f ∈Lq−p+1, f ≥ 0. (iii) For every compact set E ⊂ G, ω(E) ≤ CC˙p, q (E). q−p+1 (iv) The inequality q [W∞ (gdω)(x)]qdx ≤ C gp−1dω ˆ 1,p ˆ G G q holds for all g ∈ Lp−1(dω), g ≥ 0. 6 NGUYENCONGPHUCANDIGORE.VERBITSKY (v) The inequality [W∞ ω (x)]qdx ≤Cω(B) ˆ 1,p B B holds for all Carnot-Carathéodory balls B ⊂ G. (vi) For all x ∈ Ω, W∞ [(W∞ ω)q](x) ≤CW∞ ω(x). 1,p 1,p 1,p Conversely, there exists a constant C = C (M,p,q) > 0 such that if any 0 0 one of the statements (ii)–(vi) holds with C ≤ C then equation (1.15) has 0 a nonnegative p-superharmonic solution u ∈ Lq (G). Moreover, u satisfies loc the following pointwise two-sided estimate κ W∞ ω ≤ u ≤ κ W∞ ω. 1 1,p 2 1,p Theorem 1.4 yields thefollowing Liouville typetheorem. We observe that for p 6= 2 this Liouville type theorem is new even in the Heisenberg group. For p = 2, as mentioned earlier, such a result was obtained in [GL], [BCC], and [PVe] in the setting of the Heisenberg group. However, the approach of using test functions and integration by parts in these papers does not seem to work in the general setting of Carnot groups of arbitrary step. Corollary 1.5. If q ≤ MM(p−−p1), then the inequality −∆G,pu ≥ uq admits no nontrivial nonnegative p-superharmonic distributional solutions in G. The proofs of Theorem 1.4 and Corollary 1.5 will be given in Sect. 5. 2. Preliminaries on Carnot groups Let G bea Lie group, i.e., a differentiable manifold endowed with a group structure such that the map G×G → G defined by (x,y) 7→ xy−1 is C∞. Here y−1 is the inverse of y and xy−1 denotes the group multiplication of x by y−1. We will denote by L (x) = x x, R (x) = xx , x0 0 x0 0 respectively, the left- and right-translations on G. A vector field X on G is called left-invariant if for each x ∈G, 0 dL (X(x)) = X(x x) x0 0 for all x ∈ G, i.e., dL ◦X = X ◦L . Here dL is the differential of L . x0 x0 x0 x0 Under the Lie bracket operation on vector fields, the set of left-invariant vector fields on G forms a Lie algebra called the Lie algebra of G and is denoted by G. Note that we can identify G with the tangent space G e to G at the identity e ∈ G via the isomorphism α : G → G defined by e α(X) =X(e) and thus dimG = dimG = N, the topological dimension of G. A Carnot group G of step r is a connected and simply connected Lie group whose Lie algebra G admits a nilpotent stratification of step r, i.e., G = V ⊕V ⊕···⊕V with [V ,V ]= V for i = 1,...,r−1, V 6= {0} and 1 2 r 1 i i+1 r [V ,V ]= 0, where [·,·] denotes the Lie bracket. 1 r QUASILINEAR EQUATIONS WITH SOURCE TERMS ON CARNOT GROUPS 7 Let {X }m be a basis for the first layer V (also called the horizontal j j=1 1 layer) of G. Then for 2 ≤ i ≤ r, we can choose a basis {X }, 1 ≤ j ≤ ij dim(V ), for V consisting of commutators of length i. In particular, X = i i 1j X for j = 1,...,m, and m = dim(V ). We then define an inner product j 1 < ·,· >onG bydeclaringtheX ’stobeorthonormal. SinceGisconnected ij and simply connected, the exponential map exp is a global diffeomorphism from G onto G (see [VSC], [Va]). Thus for each x ∈ G, there is a unique xˆ = (x ) ∈ RN, 1 ≤ i ≤ r, 1 ≤ j ≤ dim(V ), and N = r dim(V ), the ij i i=1 i topological dimension of G, such that P x = exp x X . ij ij Thus the maps φ : G → R, 1 ≤ i≤(cid:16)Xr, 1 ≤ j ≤(cid:17)dim(V ), defined by ij i φ (x) = x for x = exp x X , ij ij ij ij form a system of global coordinates on G w(cid:16)hXich are ca(cid:17)lled the exponential coordinates. Henceforth we will always use these coordinates and simply write x = (φ (x)) = (x ) for x = exp x X . ij ij ij ij Such an identification of G with its Lie algebr(cid:16)aXis justified(cid:17)by the Baker- Cambell-Hausdorff formula (see, e.g., [Va]) exp x X exp y X = exp H x X , y X , ij ij ij ij ij ij ij ij where H(cid:16)X(X,Y) = (cid:17)X +(cid:16)YX+ 1[X,Y(cid:17)]+··· whith(cid:16)Xthe dots inXdicating a(cid:17)ifinite 2 linearcombinationoftermscontainingcommutatorsofordertwoandhigher. If we define a group law ∗ on G by X ∗Y = H(X,Y) then the group G can be identified with (G,∗) via the exponential coordinates. Note that from the Baker-Cambell-Hausdorff formula we have φ (x x) =φ (x )+φ (x)+P (x ,x), ij 0 ij 0 ij ij 0 where P (x ,x) depends only on the coordinates φ (x ) and φ (x) with ij 0 kl 0 kl k < i. Thus the determinant of dL is equal to 1, and the same properties x0 hold for the right translation R and its differential dR as well. It follows x0 x0 that Lebesgue measure on G is lifted via the exponential mapping exp to a bi-invariant Haar measure on G, which we will denote by dx. For a given function f : G → R, the action of X ∈ G on f is specified by the equation f(xexp(tX))−f(x) d Xf(x)= lim = f(xexp(tX))| . t=0 t→0 t dt For t > 0, we define the dilation δ : G → G by t δ (x)= (tiφ (x)) t ij 8 NGUYENCONGPHUCANDIGORE.VERBITSKY whose Jacobian determinant is everywhere equal to tM, where r M = i dim(V ) i i=1 X isthehomogeneous dimensionofG. Ahomogeneous norm|·|onGisdefined by 1/2r! |x| = |φ (x)|2r!/i , ij which obviously satisfies |δ (x(cid:16))X| = t|x| and |x−(cid:17)1| = |x|. This homogeneous t norm generates a quasi-metric ρ(x,y) = |x−1y| equivalent to the Carnot- Carathéodory metric d on G (see [NSW], [VSC]). Here cc b d (x,y) = inf < γ˙(t),γ˙(t)>dt, cc γ ˆ a p wherethe infimum is taken over all curves γ : [a,b] → G such that γ(a) = x, γ(b) = y and γ˙(t) ∈ V for all t. Such a curve is called a horizontal curve 1 connectingx,y ∈ G. By Chow-Rashevsky’s accessibility theorem (see [Cho], [Ra]), any two points x,y ∈ G can be joined by a horizontal curve of finite length and hence d is a left-invariant metric on G. We will denote by cc B (x) = {y ∈ G : d (x,y) <R} R cc theCarnot-Carathéodory metricballcenteredatxwithradiusR. Notethat there is c= c(G) such that |B (x)| = cRM, R where for a Borel set E ⊂ G we write |E| for dx. Moreover, by homo- E geneity and left-invariance we have ´ |δ (E)| = tM|E|, d(δ (x)) = tMdx, t t and for x,x′,y ∈G, d (yx,yx′) = d (x,x′), B (x) = xB (e). cc cc R R 3. p-superharmonic functions on Carnot groups Let p > 1 and let Ω be an open set in G. Recall from the previous section that X = (X ,X ,...,X ) = (X ,X ,...,X ) is an orthonormal basic 1 2 m 11 12 1m for the first layer V of G. The horizontal Sobolev space S1,p(Ω) associated 1 with the system X is defined by S1,p(Ω)= {u ∈ Lp(Ω): X u∈ Lp(Ω), i =1,...,m}, i where X u is understood in the sense of distributions, i.e., i X u(ϕ) = − uX ϕdx i ˆ i Ω QUASILINEAR EQUATIONS WITH SOURCE TERMS ON CARNOT GROUPS 9 for every ϕ ∈C∞(Ω). It is a Banach space equipped with the norm 0 1 ||u|| = (|u|p +|Xu|p) p. S1,p(Ω) ˆ Ω (cid:16) (cid:17) 1,p The corresponding local Sobolev space S (Ω) is defined similarly, with loc Lp (Ω) in place of Lp(Ω). We will denote by S1,p(Ω) the completion of loc 0 C∞(Ω) under the norm ||·|| . 0 S1,p(Ω) Recall that for a smooth function u on G, the p-Laplacian of u is defined by m ∆G,pu= Xi(|Xu|p−2Xiu), i=1 X where Xu = X uX +X uX +···+X uX is the horizontal gradient of 1 1 2 2 m m u, and |Xu|2 = m |X u|2. A function u ∈ S1,p(Ω) is said to be a weak i=1 i loc solution to P (3.1) ∆G,pu = 0 if |Xu|p−2Xu·Xϕdx = 0 ˆ Ω foreveryϕ ∈ C∞(Ω). HereXu·Xϕ = m X uX ϕ. Itisknownthatevery 0 i=1 i i weak solution to (3.1) has a continuous representative (see [TW], [HKM]), P and such continuous solutions are called p-harmonic functions on Ω. On the 1,p other hand, if u∈ S (Ω) and loc |Xu|p−2Xu·Xϕdx ≥ 0 ˆ Ω for every ϕ ∈C∞(Ω), ϕ≥ 0 then u is called a supersolution to (3.1). 0 Alowersemicontinuousfunctionu :Ω → (−∞,∞]iscalledp-superharmonic if u is not identically infinite in each component of Ω, and if for all open sets D such that D ⊂ Ω, and all functions v ∈ C(D), p-harmonic in D, it follows that v ≤ u on ∂D implies v ≤ u in D. The following fundamental connection between supersolutions to (3.1) and p-superharmonic functions can be found in [TW]. Proposition 3.1. Let u∈ S1,p(Ω) be a supersolution to (3.1). Let loc u(x) = essliminfu(y). y→x Then u is p-superharmonic and u= u a.e. From this proposition it follows that we may assume all supersolutions to be lower semicontinuous. Therefore a function u is a supersolution to (3.1) 1,p if and only if u is p-superharmonic and belongs to S (Ω). loc Note that a p-superharmonic function u does not necessarily belong to 1,p S (Ω), but its truncation min{u,k} does for every integer k. Using this loc 10 NGUYENCONGPHUCANDIGORE.VERBITSKY we set Xu = lim X[min{u,k}], k→∞ defined a.e. If either u ∈ L∞(Ω) or u ∈ S1,1(Ω), then Xu coincides with loc the regular distributional horizontal gradient of u. In general we have the following gradient estimate [TW] (see also [HKM]). Proposition 3.2 ([TW]). Suppose u is p-superharmonic in Ω. Then Xu belongs to Lr (Ω), where r < M(p−1). loc M−1 From Proposition 3.2 and the dominated convergence theorem we have |Xu|p−2Xu·Xϕdx = lim |Xu |p−2Xu ·Xϕdx ≥0 ˆΩ k→∞ˆΩ k k whenever ϕ ∈C∞(Ω) and ϕ ≥ 0, where u = min{u,k}. Thus the map 0 k ϕ 7→ |Xu|p−2Xu·Xϕdx ˆ Ω is a nonnegative distribution in Ω for a p-superharmonic function u. It follows that there is a positive (not necessarily finite) Radon measure denoted by µ[u] such that |Xu|p−2Xu·Xϕdx = ϕdµ[u], ∀ϕ∈ C∞(Ω), ˆ ˆ 0 Ω Ω or in short we write −∆G,pu= µ[u] in Ω. The close relation between p-superharmonic functions and measures gen- eratedbythemisestablishedintheweakcontinuitytheoremduetoTrudinger and Wang [TW]. Theorem 3.3 ([TW]). Suppose that {u } is a sequence of nonnegative p- n superharmonic functions in Ω that converges a.e. to a p-superharmonic function u. Then the sequence of corresponding measures {µ[u ]} converges n to µ[u] weakly, i.e., lim ϕdµ[u ]= ϕdµ[u], n→∞ˆ n ˆ Ω Ω for all ϕ ∈C∞(Ω). 0 ThefollowingpointwiseestimatesbymeansofWolff’spotentialswerealso proved in [TW] which extend earlier results due to Kilpeläinen and Maly´ [KM2] to the subelliptic setting. They will play an essential role in this paper. Theorem 3.4 ([TW]). Suppose u ≥ 0 is a p-superharmonic function in B3r(x). If µ = −∆G,pu, then (3.2) C Wr µ(x) ≤ u(x) ≤ C inf u+C W2r µ(x), 1 1,p 2 3 1,p Br(x)