Quantum Graphs I. Some Basic Structures


 Joshua Campbell
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1 Quantum Graphs I. Som Basic Structurs Ptr Kuchmnt Dpartmnt of Mathmatics Txas A& M Univrsity Collg Station, TX, USA 1 Introduction W us th nam quantum graph for a graph considrd as a ondimnsional singular varity and quippd with a slfadjoint diffrntial (in som cass psudodiffrntial) oprator ( Hamiltonian ). Thr ar manifold rasons for studying quantum graphs. Thy naturally aris as simplifid (du to rducd dimnsion) modls in mathmatics, physics, chmistry, and nginring (nanotchnology), whn on considrs propagation of wavs of diffrnt natur (lctromagntic, acoustic, tc.) through a msoscopic quasiondimnsional systm that looks lik a thin nighborhood of a graph. On can mntion in particular th frlctron thory of conjugatd molculs in chmistry, quantum chaos, quantum wirs, dynamical systms, photonic crystals, scattring thory, and a varity of othr applications. W will not discuss any dtails of ths origins of quantum graphs, rfrring th radr instad to [54] for a rcnt survy and litratur. Th problms addrssd in th quantum graph thory includ justifications of quantum graphs as approximations for mor ralistic (and complx) modls of wavs in complx structurs, analysis of various dirct and invrs spctral problms (coming from quantum chaos, optics, scattring thory, and othr aras), and many othrs. This papr dos not contain discussion of most of ths topics and th radr is rfrrd to th survy [54] and to paprs prsntd in th currnt issu of Wavs in Random Mdia for mor information and rfrncs. In this papr w addrss som basic notions and rsults concrning quantum graphs and thir spctra. Whil th spctral thory of combinatorial graphs is a rathr wll stablishd topic (.g., books [12, 21, 22, 23, 62] and 1
2 rfrncs thrin), th corrsponding thory of quantum graphs is just dvloping (.g., xampls of such studis in [3, 5, 6, 7, 8, 9, 15], [16]  [20], [25], [29]  [35], [39], [43]  [49], [56]  [58], [64, 65, 70, 74, 79, 80] and furthr rfrncs in [54]). Lt us dscrib th contnts of th following sctions. Sction 2 is dvotd to introducing basic notions of a mtric and quantum graph. Th largst Sction 3 dals with th dtaild dscription of slfadjoint vrtx conditions for scond drivativ Hamiltonians on quantum graphs. Tratmnt of infinit graphs rquird som rstrictions on thir structur. Th vrtx conditions ar writtn in th form that nabls on to dscrib asily th quadratic forms of th oprators and to classify all prmutationinvariant conditions. Sction 4 is dvotd to rlations btwn quantum and combinatorial spctral problms that will b sn as spcially hlpful in th plannd nxt part [56] of this articl. Th papr nds with short sctions containing rmarks and acknowldgmnts. Th radr should not that this papr is of th survy natur and hnc most of th rsults ar not nw (although th xposition might diffr from othr sourcs). Som rfrncs ar providd throughout th txt, albit th bibliography was not intndd to b comprhnsiv, and th radr is dirctd to th survys [54, 55] for mor dtaild bibliography. It was initially plannd to addrss svral nw issus in this papr, among which on can mntion abov all a mor dtaild spctral tratmnt of infinit graphs (bounds for gnralizd ignfunctions, Schnol s thorms, priodic graphs, som gap opning ffcts, and discussion of bound stats), howvr th articl siz limitations rsultd in th ncssity of postponing thos to th nxt papr [56]. For th sam rason, th author has also rstrictd considrations to th cas of th scond drivativ Hamiltonian only, whil on can xtnd ths without much of a difficulty to mor gnral Schrödingr oprators (.g., [46]). This, as wll as som othr topics will b dalt with lswhr. So, th papr is plannd to srv as an introduction that could b usful whil rading othr articls of this issu of Wavs in Random Mdia, and also as th first part of [56]. 2 Quantum graphs As it was mntiond in th introduction, w will b daling with quantum graphs, i.. graphs considrd as ondimnsional singular varitis rathr 2
3 than purly combinatorial objcts and corrspondingly quippd with diffrntial (or somtims psudodiffrntial ) oprators (Hamiltonians) rathr than discrt Laplac oprators. 2.1 Mtric graphs A graph Γ consists of a finit or countably infinit st of vrtics V = {v i } and a st E = { j } of dgs conncting th vrtics. Each dg can b idntifid with a pair (v i, v k ) of vrtics. Although in many quantum graph considrations dirctions of th dgs ar irrlvant and could b fixd arbitrarily (w will not nd thm in this papr), it is somtims mor convnint to hav thm assignd. Loops and multipl dgs btwn vrtics ar allowd, so w avoid saying that E is a subst of V V. W also dnot by E v th st of all dgs incidnt to th vrtx v (i.., containing v). It is assumd that th dgr (valnc) d v = E v of any vrtx v is finit and positiv. W hnc xclud vrtics with no dgs coming in or going out. This is natural, sinc for th quantum graph purposs such vrtics ar irrlvant. So far all our dfinitions hav dalt with a combinatorial graph. Hr w introduc a notion that maks Γ a topological and mtric objct. Dfinition 1. A graph Γ is said to b a mtric graph (somtims th notion of a wightd graph is usd instad), if its ach dg is assignd a positiv lngth l (0, ] (notic that dgs of infinit lngth ar allowd). Having th lngth assignd, an dg will b idntifid with a finit or infinit sgmnt [0, l ] of th ral lin with th natural coordinat x along it. In most cass w will drop th subscript in th coordinat and call it x, which should not lad to any confusion. This nabls on to intrprt th graph Γ as a topological spac (simplicial complx) that is th union of all dgs whr th nds corrsponding to th sam vrtx ar idntifid. Th radr should not that w do not assum th graph to b mbddd in any way into an Euclidan spac. In som applications such a natural mbdding dos xist (.g., in modling quantum wir circuits or photonic crystals), and in such cass th coordinat along an dg is usually th arc lngth. In som othr applications (.g., in quantum chaos) th graph is not assumd to b mbddd. Graph Γ can b quippd with a natural mtric. If a squnc of dgs { j } M j=1 forms a path, its lngth is dfind as l j. For two vrtics v and w, th distanc ρ(v, w) is dfind as th minimal path lngth btwn thm. 3
4 Sinc along ach dg th distanc is dtrmind by th coordinat x, it is asy to dfin th distanc ρ(x, y) btwn two points x, y of th graph that ar not ncssarily vrtics. W lav this to th radr. W also impos som additional conditions: Condition A. Th infinit nds of infinit dgs ar assumd to hav dgr on. Thus, th graph can b thought of as a graph with finit lngth dgs with additional infinit lads or nds going to infinity attachd to som vrtics. This situation ariss naturally for instanc in scattring thory. Sinc ths infinit vrtics will nvr b tratd as rgular vrtics (in fact, in this papr such vrtics will not aris at all), on can just assum that ach infinit dg is a ray with a singl vrtx. Condition B. Whn studying infinit graphs, w will impos som assumptions that will imply in particular that for any positiv numbr r and any vrtx v thr is only a finit st of vrtics w at a distanc lss than r from v. In particular, th distanc btwn any two distinct vrtics is positiv, and thr ar no finit lngth paths of infinitly many dgs. This obviously mattrs only for infinit graphs (i.., graphs with infinitly many dgs) and is automatically satisfid for th class of infinit mtric graphs that will b introducd latr. So, now on can imagin th graph Γ as a ondimnsional simplicial complx, ach 1D simplx (dg) of which is quippd with a smooth structur, with singularitis arising at junctions (vrtics) (s Fig. 1). Th radr should notic that now th points of th graph ar not only its vrtics, but all intrmdiat points x on th dgs as wll. On can dfin in th natural way th Lbsgu masur dx on th graph. Functions f(x) on Γ ar dfind along th dgs (rathr than at th vrtics as in discrt modls). Having this and th masur, on can dfin in a natural way som function spacs on th graph: Dfinition Th spac L 2 (Γ) on Γ consists of functions that ar masurabl and squar intgrabl on ach dg and such that f 2 L 2 (Γ) = E f 2 L 2 () <. In othr words, L 2 (Γ) is th orthogonal dirct sum of spacs L 2 (). 2. Th Sobolv spac H 1 (Γ) consists of all continuous functions on Γ 4
5 Figur 1: Graph Γ. that blong to H 1 () for ach dg and such that f 2 H 1 () <. E Not that continuity in th dfinition of th Sobolv spac mans that th functions on all dgs adjacnt to a vrtx v assum th sam valu at v. Thr sm to b no natural dfinition of Sobolv spacs H k (Γ) of ordr k highr than 1, sinc boundary conditions at vrtics dpnd on th Hamiltonian (s dtails latr on in this papr). Th last stp that is ndd to finish th dfinition of a quantum graph is to introduc a slfadjoint (diffrntial or mor gnral) oprator (Hamiltonian) on Γ. This is don in th nxt sction. 2.2 Oprators Th oprators of intrst in th simplst cass ar: th ngativ scond drivativ a mor gnral Schrödingr oprator f(x) d2 f dx 2, (1) 5
6 f(x) d2 f + V (x)f(x), (2) dx2 or a magntic Schrödingr oprator ( ) 2 1 d f(x) i dx A(x) + V (x)f(x). (3) Hr x dnots th coordinat x along th dg. Highr ordr diffrntial and vn psudodiffrntial oprators aris as wll (s,.g. th survy [54] and rfrncs thrin). W, howvr, will concntrat hr on scond ordr diffrntial oprators, and for simplicity of xposition spcifically on (1). In ordr for th dfinition of th oprators to b complt, on nds to dscrib thir domains. Th natural conditions rquir that f blongs to th Sobolv spac H 2 () on ach dg. On also clarly nds to impos boundary valu conditions at th vrtics. Ths will b studid in th nxt sction. 3 Boundary conditions and slfadjointnss W will discuss now th boundary conditions on would lik to add to th diffrntial xprssion (1) in ordr to crat a slfadjoint oprator. 3.1 Graphs with finitly many dgs In this sction w will considr finit graphs only. This mans that w assum that th numbr of dgs E is finit (and hnc th numbr of vrtics V is finit as wll, sinc w assum all vrtx dgrs to b positiv). Notic that dgs ar still allowd to hav infinit lngth. W will concntrat on local (or vrtx) boundary conditions only, i.. on thos that involv th valus at a singl vrtx only at a tim. It is possibl to dscrib all th vrtx conditions that mak (1) a slfadjoint oprator (s [43, 41] and a partial dscription in [34]). This is don by ithr using th standard von Numann thory of xtnsions of symmtric oprators (as for instanc dscribd in [1]), or by its mor rcnt vrsion that amounts to finding Lagrangian plans with rspct to th complx symplctic boundary form that corrsponds to th maximal oprator (s for instanc 6
7 [3, 26, 27, 28, 41, 42, 43, 69, 71] for th accounts of this approach that gos back at last as far as [63], whr it was prsntd without us of words symplctic or Lagrangian ). On of th most standard typs of such boundary conditions is th Kirchhoff condition: f(x) is continuous on Γ and at ach vrtx v on has E v df dx (v) = 0, (4) whr th sum is takn ovr all dgs containing th vrtx v. Hr th drivativs ar takn in th dirctions away from th vrtx (w will call ths outgoing dirctions ), th agrmnt w will adhr to in all cass whn ths conditions ar involvd. Somtims (4) is calld th Numann condition. It is clar that at loos nds (vrtics of dgr 1) it turns into th actual Numann condition. Bsids, as th Numann boundary condition for Laplac oprator, it is natural. Namly, as it will b sn a littl bit latr, th domain of th quadratic form of th corrsponding oprator dos not rquir any conditions on a function bsids bing in H 1 (Γ) (and hnc continuous). It is also usful to not that undr th boundary conditions (4) on can liminat all vrtics of dgr 2, conncting th adjacnt dgs into on smooth dg. Thr ar many othr plausibl vrtx conditions (som of which will b discussd latr), and th qustion w want to addrss now is how to dscrib all of thos that lad to a slfadjoint ralization of th scond drivativ along th dgs. Sinc w ar intrstd in local vrtx conditions only, it is clar that it is sufficint to addrss th problm of slfadjointnss for a singl junction of d dgs at a vrtx v. Bcaus along ach dg our oprator acts as th (ngativ) scond drivativ, on nds to stablish two conditions pr an dg, and hnc at ach vrtx th numbr of conditions must coincid with th dgr d of th vrtx. For functions in H 2 on ach dg, th conditions may involv only th boundary valus of th function and its drivativ. Thn th most gnral form of such (homognous) condition is A v F + B v F = 0. (5) Hr A v and B v ar d d matrics, F is th vctor (f 1 (v),..., f d (v)) t of th vrtx valus of th function along ach dg, and F = (f 1(v),..., f d (v))t 7
8 is th vctor of th vrtx valus of th drivativs takn along th dgs in th outgoing dirctions at th vrtx v, as w hav agrd bfor. Th rank of th d 2d matrix (A v, B v ) must b qual to d (i.., maximal) in ordr to nsur th corrct numbr of indpndnt conditions. Whn this would not lad to confusion, w will drop th subscript v in ths matrix notations, rmmbring that th matrics dpnd on th vrtx (in fact, for nonhomognous graphs thy ssntially hav no othr choic). Now on is intrstd in th ncssary and sufficint conditions on matrics A and B in (5) that would guarant slfadjointnss of th rsulting oprator. All such conditions wr compltly dscribd in [43] (s also th arlir papr [33] for som spcial cass and [41, 48]for an altrnativ considration that rprsnts th boundary conditions in trms of vrtx scattring matrics). W will formulat th corrsponding rsult in th form takn from [43]. Thorm 3. [43] Lt Γ b a mtric graph with finitly many dgs. Considr th oprator H acting as d2 dx 2 on ach dg, with th domain consisting of functions that blong to H 2 () on ach dg and satisfy th boundary conditions (5) at ach vrtx. Hr {A v, B v v V } is a collction of matrics of sizs d v d v such that ach matrix (A v B v ) has th maximal rank. In ordr for H to b slfadjoint, th following condition at ach vrtx is ncssary and sufficint: th matrix A v B v is slfadjoint. (6) Th proof of this thorm can b found in [43]. W would lik now to dscrib th quadratic form of th oprator H corrsponding to th (ngativ) scond drivativ along ach dg, with slfadjoint vrtx conditions (5) (w assum in particular that (6) is satisfid). In ordr to do so, w will stablish first a coupl of simpl auxiliary statmnts. In th nxt two lmmas and a corollary w will considr matrics A and B as in (8). Sinc w will b concrnd with a singl vrtx hr, w will drop for this tim th subscripts v in A v, B v, and d v. Lt us introduc som notations. W will dnot by P and P 1 th orthogonal projctions in C d onto th krnls K = kr B and K 1 = kr B rspctivly. Th complmntary orthogonal projctors onto th rangs R = R(B ) and R 1 = R(B) ar dnotd by Q and Q 1 (hr R(M) dnots th rang of a matrix M). 8
9 Lmma 4. Lt d d matrics A and B b such that th d 2d matrix (A B) has maximal rank and AB is slfadjoint. Thn 1. Oprator A maps th rang R of B into th rang R 1 of B. 2. Th mapping P 1 AP : K K 1 is invrtibl. 3. Th mapping Q 1 BQ : R R 1 is invrtibl (w dnot its invrs by B ( 1) ). 4. Th matrix B ( 1) AQ is slfadjoint. Proof. Slfadjointnss of AB mans AB = BA, which immdiatly implis th first statmnt of th lmma. In ordr to prov th nxt two statmnts, lt us dcompos th spac C d into th orthogonal sum R 1 K 1 and C 2d into C d C d = R K R K. Thn th matrix (A B) rprsnting an oprator from C 2d into C d can b writtn in a 2 4 blockmatrix form with rspct to ths dcompositions. Taking into account th dfinitions of th subspacs R, R 1, K, and K 1 and th alrady provn first statmnt of th lmma, this lads to th block matrix (A B) = ( ) A11 A 12 B (7) 0 A For this matrix to hav maximal rank, th ntry A 22 must b invrtibl, which givs th scond statmnt of th lmma. Th third statmnt is obvious, sinc th matrix B 11 is squar and has no krnl (which has alrady bn liminatd and includd into K). Immdiat calculation shows that slfadjointnss of AB mans that th squar matrix A 11 B11 is slfadjoint, i.. A 11 B11 = B 11 A 11. Sinc invrtibility of B 11 has alrady bn stablishd (rcall that its invrs is dnotd by B ( 1) ), w can multiply th prvious quality by appropriat invrs matrics from both sids to gt B ( 1) A 11 = A 11B ( 1). This mans that th matrix B ( 1) A 11 is slfadjoint and hnc th last statmnt of th lmma is provn. Corollary 5. Lt th conditions of Lmma 4 b satisfid. Thn th boundary condition (5) AF + BF = 0 is quivalnt to th pair of conditions P F = 0 9
10 and LQF + QF = 0, whr P, as bfor, is th orthogonal projction onto th krnl of matrix B, Q is th complmntary projctor, and L is th slfadjoint oprator B ( 1) A. Proof. W will mploy th notations usd in th prcding lmma. It is clar that (5) is quivalnt to th pair of conditions P 1 AF + P 1 BF = 0 and Q 1 AF + Q 1 BF = 0. Th lmma now shows that th first of thm can b rwrittn as A 22 P F = 0, which by th scond statmnt of th lmma is quivalnt to P F = 0. Th scond quality, again by th lmma, can b quivalntly rwrittn as AQF + B 11 QF = 0, or aftr invrting B 11 as LQF + QF = 0, which finishs th proof of th corollary. W can now rphras gnral slfadjoint boundary conditions in a fashion that is somtims mor convnint (for instanc, for dscribing th quadratic form of th oprator). Thorm 6. All slfadjoint ralizations H of th ngativ scond drivativ on Γ with vrtx boundary conditions can b dscribd as follows. For vry vrtx v thr ar an orthogonal projctor P v in C dv with th complmntary projctor Q v = Id P v and a slfadjoint oprator L v in Q v C dv. Th functions f from th domain D(H) H 2 () of H ar dscribd by th following conditions at ach (finit) vrtx v: P v F (v) = 0 Q v F (v) + L v Q v F (v) = 0. (8) In trms of th matrics A v and B v of Thorm 3, P v is th orthogonal projctor onto th krnl of B v and L v = B v ( 1) A v (whr B v ( 1) has bn dfind prviously). Proof. Adopting th dfinitions of P v and L v providd in th thorm, on can s that th thorm s statmnt is just a simpl consqunc of Thorm 3, Lmma 4, and Corollary 5 combind. Rmark In viw of th first condition in (8), th scond on can b quivalntly writtn as Q v F (v) + L v F (v) = Conditions (8) say that th P v componnt of th vrtx valus F (v) of f must b zro (kind of a Dirichlt part), whil th P v part of th drivativs F (v) is unrstrictd. Th Q v part of th drivativs F (v) is dtrmind by th Q v part of th function F (v). 10
11 W will nd also th following wll known trac stimat that w prov hr for compltnss. Lmma 8. Lt f H 1 [0, a], thn for any l a. f(0) 2 2 l f 2 L 2 [0,a] + l f 2 L 2 [0,a] (9) Proof. Du to H 1 continuity of both sids of th inquality, it is sufficint to prov it for smooth functions. Start with th rprsntation f(0) = f(x) and stimat by CauchySchwartz inquality This implis x 0 x 0 f (t)dt, x [0, l] (10) f (t)dt 2 f 2 L 2 [0,a] χ [0,x] 2 L 2 [0,a] = x f 2 L 2 [0,a]. x f (t)dt 2 L 2 [0,l] f 2 L 2 [0,a] l x dx = l2 2 f 2 L 2 [0,a]. 0 0 Now taking L 2 [0, l]norms in both sids of (10) and using triangl inquality and (a + b) 2 2a 2 + 2b 2, w gt th stimat f(0) 2 l 2 f 2 L 2 [0,a] + l 2 f 2 L 2 [0,a], which implis th statmnt of th lmma. W ar rady now for th dscription of th quadratic form of th oprator H on a finit graph Γ. Lt as bfor Γ b a mtric graph with finitly many vrtics. Th slfadjoint oprator H in L 2 (Γ) acts as d2 along ach dg, dx2 with th domain consisting of all functions f(x) on Γ that blong to th 11
12 Sobolv spac H 2 () on ach dg and satisfy at ach vrtx v conditions (8): P v F (v) = 0 Q v F (v) + L v Q v F (v) = 0. (11) Hr, as always F (v) = (f 1 (v),...f dv (v)) t is th column vctor of th valus of th function f at v that it attains whn v is approachd from diffrnt dgs j adjacnt to v, F (v) is th column vctor of th corrsponding outgoing drivativs at v, th d v d v matrix P v is an orthogonal projctor and L v is a slfadjoint oprator in th krnl Q v C d v of P v. Thorm 9. Th quadratic form h of H is givn as h[f, f] = df dx 2 dx (L v ) jk f j (v)f k (v) E v V j, k E v = df dx 2 dx L v F, F, E v V (12) whr, dnots th standard hrmitian innr product in C d. Th domain of this form consists of all functions f that blong to H 1 () on ach dg and satisfy at ach vrtx v th condition P v F = 0. Corrspondingly, th ssquilinar form of H is: h[f, g] = E df dg dx dx dx L v F, G. (13) v V Proof. Notic that Lmma 8 shows that (12) with th domain dscribd in th thorm dfins a closd quadratic form. It hnc corrsponds to a slfadjoint oprator M in L 2 (Γ). Intgration by parts in (13) against smooth functions g that vanish in a nighborhood of ach vrtx shows that on its domain M acts as th ngativ scond drivativ along ach dg. So, th rmaining task is to show that its domain D(M) consists of all functions that blong to H 2 on ach dg and satisfy th vrtx conditions (8). This would imply that M = H. So, lt us assum f D(M). In particular, f H 1 (). It is th standard conclusion thn that f H 2 () for any dg (w lav to th radr to fill in th dtails, s also th sction concrning infinit graphs). W nd now to vrify that f satisfis th vrtx conditions (8). Th condition P v F (v) = 0 dos not nd to b chckd, sinc 12
13 it is satisfid on th domain of th quadratic form. transforms (13) into Intgration by parts E d 2 f dx gdx F + L 2 v F, G. (14) v V Th scond trm must vanish for any g in th domain of th quadratic form. Taking into account that thn G(v) can b an arbitrary vctor such that P v G(v) = 0, this mans that for ach v th quality Q v F (v) + Q v L v F (v) = 0 (15) nds to b satisfid, whr Q v is th complmntary projction to P v. This givs us th ndd conditions (8) for th function f. It is also asy to chck in a similar fashion that as soon as a function f blongs to H 2 on ach dg and satisfis (8), it blongs to th domain of M. This provs that M in fact coincids with th prviously dscribd oprator H. Th proof is hnc compltd. Corollary 10. Th oprator H is boundd from blow. Morovr, lt S = max{ L v }, thn v whr H C Id, (16) C = 4S max{2s, max{l ( 1) }}. Proof. On can choos l := min{l } in (9) applid to any dg. Thn, du to (9) on has: v L v F (v), F (v) S ( v 2 l f 2 L 2 (Γ) + l f 2 L 2 (Γ) 2S F (v) 2 ). (17) If now 2lS 1, thn (17) and th dfinition of th quadratic form h show that th statmnt of th Corollary holds. Although on can (and oftn nds to) considr quantum graphs with mor gnral Hamiltonian oprators (.g. Schrödingr oprators with lctric and magntic potntials, oprators of highr ordr, psudodiffrntial oprators, tc.), for th purpos of this articl only w adopt th following dfinition: 13
14 Dfinition 11. A quantum graph is a mtric graph quippd with th oprator H that acts as th ngativ scond ordr drivativ along dgs and is accompanid by th vrtx conditions (8). 3.2 Exampls of boundary conditions In this sction w tak a brif look from th prospctiv of th prvious sction at som xampls of vrtx conditions and corrsponding oprators. Th radr can find mor xampls in [34, 43, 59] δtyp conditions ar dfind as follows: and at ach vrtx v, f(x) is continuous on Γ E v df dx (v) = α v f(v). (18) Hr α v ar som fixd numbrs. On can rcogniz ths conditions as an analog of conditions on obtains from a Schrödingr oprator on th lin with a δ potntial, which xplains th nam. In this cas th conditions can b obviously writtn in th form (5) with and Sinc A v = α v B v = A v Bv = , α v th slfadjointnss condition (6) is satisfid if and only if α is ral. 14
15 In ordr to writ th vrtx conditions in th form (8), on nds to find th orthogonal projction P v onto th krnl of B v and th slfadjoint oprator L v = B v ( 1) A v Q v. It is a simpl xrcis to find that Q v is th ondimnsional projctor onto th spac of vctors with qual coordinats and corrspondingly th rang of P v is spannd by th vctors r k, k = 1,..., d v 1, whr r k has 1 as th kth componnt, 1 as th nxt on, and zros othrwis. Thn a straightforward calculation shows that L v is th multiplication by th numbr α v. In particular, th dscription of th projctor P v shows d v that th quadratic form of th oprator H is dfind on functions that ar continuous throughout all vrtics (i.., F (v) = (f(v),..., f(v)) t ) and hnc blong to H 1 (Γ). Th form is computd as follows: df dx 2 dx L v F, F E v V = df dx 2 dx + α v f(v) 2 (19). E v V It is obvious from (19) that th oprator is nonngativ if α v 0 for all vrtics v Numann (Kirchhoff) conditions Ths conditions (4) that hav alrady bn mntiond, rprsnt probably th most common cas of th δtyp conditions (18) whn α l = 0, i.. f(x) is continuous on Γ and at ach vrtx v, E v df dx (v) = 0. (20) Th discussion abov shows that th quadratic form of H is df dx 2 dx, (21) E dfind on H 1 (Γ), and th oprator is nonngativ Conditions of δ typ Ths conditions rmind th δtyp ons, but with th rols of functions and th drivativs ar rvrsd at ach vrtx (s also [2]). In ordr to dscrib 15
16 thm, lt us introduc th notation f v for th rstriction of a function f onto th dg. Thn th conditions at ach vrtx v can b dscribd as follows: Th valu of th drivativ df dx (v) is th sam for all dgs E v and df f (v) = α v (v). dx E v (22) Hr, as bfor, df dx (v) is th drivativ in th outgoing dirction at th vrtx v. It is clar that in comparison with δtyp cas th matrics A v and B v ar switchd: and Sinc B v = α v A v = A v Bv = , α v th slfadjointnss condition (6) is satisfid again for ral α v only. Considr first th cas whn α v = 0 for som vrtx v. Thn th krnl of B v consists of all vctor with qual coordinats, and th projctor Q v projcts orthogonally onto th subspac of vctors that hav th sum of thir coordinats qual to zro. On this subspac oprator A v is qual to zro, and hnc L v = 0. This lads to no nonintgral contribution to th quadratic form coming from th vrtx v. In particular, if α v = 0 for all vrtics, w gt th quadratic form df dx 2 dx 16
17 with th domain consisting of all functions from H 1 () that hav at ach vrtx th sum of th vrtx valus along all ntring dgs qual to zro. In this cas th oprator is clarly nonngativ. Lt us look at th cas whn for a vrtx v th valu α v is nonzro. In this cas th oprator B v is invrtibl and so P v = 0, Q v = Id. It is not hard to comput that (L v ) ij = (αd) ( 1) for all indics i, j. This lads to th nonintgral trm 1 f (v) 2. α v { E v } On can think that th cas whn α v = 0 is formally a particular cas of this on, if on assums that th dnominator bing qual to zro forcs th condition that th sum in th numrator also vanishs. Th quadratic form for a gnral choic of ral numbrs α can b writtn as follows: E df dx 2 dx + {v V α v 0} 1 α v { E v} f (v) 2. Th domain consists of all functions in H 1 () that hav at ach vrtx v whr α v = 0 th sum of th vrtx valus along all ntring dgs qual to zro. Whn all numbrs α v ar nonngativ, th oprator is clarly nonngativ as wll Vrtx Dirichlt and Numann conditions Th vrtx Dirichlt conditions ar thos whr at ach vrtx it is rquird that th boundary valus of th function on ach dg ar qual to zro. In this cas th oprator compltly dcoupls into th dirct sum of th ngativ scond drivativs with Dirichlt conditions on ach dg. Thr is no communication btwn th dgs. Th quadratic form is clarly E df dx 2 dx on functions f H ( Γ) with th additional condition f(v) = 0 for all vrtics v. Th spctrum σ(h) is thn found as σ(h) = {n 2 π 2 /l 2 E, n Z 0}. 17
18 Anothr typ of conditions undr which th dgs compltly dcoupl and th spctrum can b asily found from th st of dg lngths, is th vrtx Numann conditions. Undr ths conditions, no rstrictions on th vrtx valus f (v) ar imposd, whil all drivativs f (v) ar rquird to b qual to zro. Thn on obtains th Numann boundary valu problm on ach dg sparatly. Th formula for th quadratic form is th sam as for th vrtx Dirichlt conditions, albit on a largr domain with no vrtx conditions imposd whatsovr Classification of all symmtric vrtx conditions Th radr might hav noticd that in all xampls abov th conditions wr invariant with rspct to any prmutations of dgs at a vrtx. W will now classify all such conditions (8). Th list of symmtric vrtx conditions includs svral popular classs. Howvr, quantum graph modls arising as approximations for thin structurs somtims involv nonsymmtric conditions as wll, which prsrv som mmory of th gomtry of junctions (s,.g. [58, 59, 60]). As it has alrady bn mntiond, on can find discussion of othr xampls of boundary conditions in [34, 43]. Lt us rpat for th radr s convninc th boundary conditions (8) at a vrtx v, dropping for simplicity of notations all subscripts indicating th vrtx: P F (v) = 0 QF (v) + LQF (v) = 0. (23) Hr, as bfor, P is an orthogonal projctor in C d, Q = I P, and L is a slfadjoint oprator in QC d. W ar now intrstd in th cas whn ths conditions ar invariant with rspct to th symmtric group S d acting on C d by prmutations of coordinats. Notic that this action has only two nontrivial invariant subspacs: th ondimnsional subspac U consisting of th vctors with qual componnts, and its orthogonal complmnt U, sinc th rprsntation of S d in U is irrducibl (.g., Sction VI.4.7 in [13] or VI.3 in [78]). Hr U consists of all vctors with th sum of componnts qual to zro. Lt us dnot by φ th unit vctor φ = (d 1/2,..., d 1/2 ) C d. This is a unit basis vctor of U. Thn th orthogonal projctor onto U is φ φ (a physicist would dnot it φ φ ) acting on a vctor a as a, φ φ. Thn th complmntary 18
19 projctor onto U is I φ φ. In ordr for (23) to b S d invariant, oprators P and L must b so. Du to th just mntiond xistnc of only two nontrivial S d invariant subspacs, thr ar only four possibl orthogonal projctors that commut with S d : P = 0, P = φ φ, P = I φ φ, and P = I. Lt us study ach of ths cass: Lt first P = 0. In this cas Q = I and L acting on C d must commut with th rprsntation of th symmtric group by prmutations of coordinats. As it was discussd abov, this implis that L = αφ φ + βi. This shows that thr ar no rstrictions imposd on th vrtx valus F and th rstrictions on F ar givn as F + α F, φ + βf = 0. In othr words, on can say that th xprssion f (v) + βf (v) is dgindpndnt and α f (v) = (f (v) + βf(v)). In th particular E v cas whn α 0, β = 0, w conclud that all th valus of th outgoing drivativs f (v) ar th sam, and (f (v)) = α ( 1) f (v). On E v rcognizs this as th δ typ conditions. If α = β = 0, on nds up with th vrtx Numann condition. Lt now P = I. Thn Q = 0 and hnc L is irrlvant. W conclud that F = 0 and no mor conditions ar imposd. This is th vrtx Dirichlt condition, undr which th dgs dcoupl. Lt P = φ φ. Thn Q = I φ φ and L is qual to a scalar α, du to irrducibility of th rprsntation in QC d. Thn E v f (v) = 0 and F (v) F (v), φ φ + αf (v) = 0. Th last quality shows that th xprssion f (v) + αf (v) is dgindpndnt and qual to f (v). This, togthr with f (v) = 0 E v E v givs all th conditions in this cas. Thr appars to b no common nam for ths conditions. Th last cas is P = I φ φ. Thn Q = φ φ and L is a scalar α again. In this cas th condition P F = 0 mans that th valus f (v) ar dg indpndnt, or in othr words f is continuous through th vrtx v. Th othr condition asily lads to f (v) = αf(v), E v which on rcognizs as th δtyp conditions. 19
20 This complts our classification of symmtric vrtx conditions. Ths conditions wr found prviously in [34] by a diffrnt tchniqu. As w hav alrady mntiond bfor, nonsymmtric conditions aris somtims as wll (.g., [34, 43, 58, 59, 60]). On of th natural qustions to considr is which of th conditions (6) aris in th asymptotic limits of problms in thin nighborhoods of graphs. This issu was discussd in [54], howvr it has not bn rsolvd yt. On might think that whn quantum graph modls ar dscribing th limits of thin domains, diffrnt typs of vrtx conditions could probably b obtaind by changing th gomtry of th domain nar th junctions around vrtics. This guss is basd in particular on th rsults of [59, 60]. 3.3 Infinit quantum graphs W will now allow th numbr of vrtics and dgs of a mtric graph Γ to b (countably) infinit. Our goal is to dfin a slfadjoint oprator H on Γ in a mannr similar to th on usd for finit graphs. In othr words, H should act as th (ngativ) scond drivativ along ach dg, and th functions from its domain should satisfy (now infinitly many) vrtx conditions (5) or quivalntly (8). This would turn a mtric graph Γ into a quantum graph. Howvr, unlss additional rstrictions on th graph and vrtx conditions ar imposd, th situation can bcom mor complx than in th finit graph cas. This is tru vn for such simpl graphs as trs, whr additional boundary conditions at infinity may or may not b ndd dpnding on gomtry (s [19, 79]). On th othr hand, if on looks at th naturally arising infinit graphs, on can notic that in many cass thr is an automorphism group acting on th graph such that th orbit spac (which is a graph by itslf) is compact. This is th cas for instanc with priodic graphs and Cayly graphs of groups. W do not nd xactly th homognity, but rathr that th gomtry dos not chang drastically throughout th graph. Th assumptions that w introduc blow capturs this ida and covrs all cass mntiond abov. It also nabls on to stablish nic proprtis of th corrsponding Hamiltonians. W would also lik to notic that this class of graphs is in som sns an analog of th so calld manifolds of boundd gomtry [76]. On such manifolds studying lliptic oprators is asir than on mor gnral ons. 20
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