$ \newcommand{\undefined}{} \newcommand{\hfill}{} \newcommand{\qedhere}{\square} \newcommand{\qed}{\square} \newcommand{\ensuremath}[1]{#1} \newcommand{\bit}{\{0,1\}} \newcommand{\Bit}{\{-1,1\}} \newcommand{\Stab}{\mathbf{Stab}} \newcommand{\NS}{\mathbf{NS}} \newcommand{\ba}{\mathbf{a}} \newcommand{\bh}{\mathbf{h}} \newcommand{\br}{\mathbf{r}} \newcommand{\bs}{\mathbf{s}} \newcommand{\bx}{\mathbf{x}} \newcommand{\by}{\mathbf{y}} \newcommand{\bz}{\mathbf{z}} \newcommand{\Var}{\mathbf{Var}} \newcommand{\dist}{\text{dist}} \newcommand{\norm}[1]{\\|#1\\|} \newcommand{\etal} \newcommand{\ie} \newcommand{\eg} \newcommand{\cf} \newcommand{\rank}{\text{rank}} \newcommand{\tr}{\text{tr}} \newcommand{\mor}{\text{Mor}} \newcommand{\hom}{\text{Hom}} \newcommand{\id}{\text{id}} \newcommand{\obj}{\text{obj}} \newcommand{\pr}{\text{pr}} \newcommand{\ker}{\text{ker}} \newcommand{\coker}{\text{coker}} \newcommand{\im}{\text{im}} \newcommand{\bbA}{\mathbb A} \newcommand{\bbB}{\mathbb B} \newcommand{\bbC}{\mathbb C} \newcommand{\bbD}{\mathbb D} \newcommand{\bbE}{\mathbb E} \newcommand{\bbF}{\mathbb F} \newcommand{\bbG}{\mathbb G} \newcommand{\bbH}{\mathbb H} \newcommand{\bbI}{\mathbb I} \newcommand{\bbJ}{\mathbb J} \newcommand{\bbK}{\mathbb K} \newcommand{\bbL}{\mathbb L} \newcommand{\bbM}{\mathbb M} \newcommand{\bbN}{\mathbb N} \newcommand{\bbO}{\mathbb O} \newcommand{\bbP}{\mathbb P} \newcommand{\bbQ}{\mathbb Q} \newcommand{\bbR}{\mathbb R} \newcommand{\bbS}{\mathbb S} \newcommand{\bbT}{\mathbb T} \newcommand{\bbU}{\mathbb U} \newcommand{\bbV}{\mathbb V} \newcommand{\bbW}{\mathbb W} \newcommand{\bbX}{\mathbb X} \newcommand{\bbY}{\mathbb Y} \newcommand{\bbZ}{\mathbb Z} \newcommand{\sA}{\mathscr A} \newcommand{\sB}{\mathscr B} \newcommand{\sC}{\mathscr C} \newcommand{\sD}{\mathscr D} \newcommand{\sE}{\mathscr E} \newcommand{\sF}{\mathscr F} \newcommand{\sG}{\mathscr G} \newcommand{\sH}{\mathscr H} \newcommand{\sI}{\mathscr I} \newcommand{\sJ}{\mathscr J} \newcommand{\sK}{\mathscr K} \newcommand{\sL}{\mathscr L} \newcommand{\sM}{\mathscr M} \newcommand{\sN}{\mathscr N} \newcommand{\sO}{\mathscr O} \newcommand{\sP}{\mathscr P} \newcommand{\sQ}{\mathscr Q} \newcommand{\sR}{\mathscr R} \newcommand{\sS}{\mathscr S} \newcommand{\sT}{\mathscr T} \newcommand{\sU}{\mathscr U} \newcommand{\sV}{\mathscr V} \newcommand{\sW}{\mathscr W} \newcommand{\sX}{\mathscr X} \newcommand{\sY}{\mathscr Y} \newcommand{\sZ}{\mathscr Z} \newcommand{\sfA}{\mathsf A} \newcommand{\sfB}{\mathsf B} \newcommand{\sfC}{\mathsf C} \newcommand{\sfD}{\mathsf D} \newcommand{\sfE}{\mathsf E} \newcommand{\sfF}{\mathsf F} \newcommand{\sfG}{\mathsf G} \newcommand{\sfH}{\mathsf H} \newcommand{\sfI}{\mathsf I} \newcommand{\sfJ}{\mathsf J} \newcommand{\sfK}{\mathsf K} \newcommand{\sfL}{\mathsf L} \newcommand{\sfM}{\mathsf M} \newcommand{\sfN}{\mathsf N} \newcommand{\sfO}{\mathsf O} \newcommand{\sfP}{\mathsf P} \newcommand{\sfQ}{\mathsf Q} \newcommand{\sfR}{\mathsf R} \newcommand{\sfS}{\mathsf S} \newcommand{\sfT}{\mathsf T} \newcommand{\sfU}{\mathsf U} \newcommand{\sfV}{\mathsf V} \newcommand{\sfW}{\mathsf W} \newcommand{\sfX}{\mathsf X} \newcommand{\sfY}{\mathsf Y} \newcommand{\sfZ}{\mathsf Z} \newcommand{\cA}{\mathcal A} \newcommand{\cB}{\mathcal B} \newcommand{\cC}{\mathcal C} \newcommand{\cD}{\mathcal D} \newcommand{\cE}{\mathcal E} \newcommand{\cF}{\mathcal F} \newcommand{\cG}{\mathcal G} \newcommand{\cH}{\mathcal H} \newcommand{\cI}{\mathcal I} \newcommand{\cJ}{\mathcal J} \newcommand{\cK}{\mathcal K} \newcommand{\cL}{\mathcal L} \newcommand{\cM}{\mathcal M} \newcommand{\cN}{\mathcal N} \newcommand{\cO}{\mathcal O} \newcommand{\cP}{\mathcal P} \newcommand{\cQ}{\mathcal Q} \newcommand{\cR}{\mathcal R} \newcommand{\cS}{\mathcal S} \newcommand{\cT}{\mathcal T} \newcommand{\cU}{\mathcal U} \newcommand{\cV}{\mathcal V} \newcommand{\cW}{\mathcal W} \newcommand{\cX}{\mathcal X} \newcommand{\cY}{\mathcal Y} \newcommand{\cZ}{\mathcal Z} \newcommand{\bfA}{\mathbf A} \newcommand{\bfB}{\mathbf B} \newcommand{\bfC}{\mathbf C} \newcommand{\bfD}{\mathbf D} \newcommand{\bfE}{\mathbf E} \newcommand{\bfF}{\mathbf F} \newcommand{\bfG}{\mathbf G} \newcommand{\bfH}{\mathbf H} \newcommand{\bfI}{\mathbf I} \newcommand{\bfJ}{\mathbf J} \newcommand{\bfK}{\mathbf K} \newcommand{\bfL}{\mathbf L} \newcommand{\bfM}{\mathbf M} \newcommand{\bfN}{\mathbf N} \newcommand{\bfO}{\mathbf O} \newcommand{\bfP}{\mathbf P} \newcommand{\bfQ}{\mathbf Q} \newcommand{\bfR}{\mathbf R} \newcommand{\bfS}{\mathbf S} \newcommand{\bfT}{\mathbf T} \newcommand{\bfU}{\mathbf U} \newcommand{\bfV}{\mathbf V} \newcommand{\bfW}{\mathbf W} \newcommand{\bfX}{\mathbf X} \newcommand{\bfY}{\mathbf Y} \newcommand{\bfZ}{\mathbf Z} \newcommand{\rmA}{\mathrm A} \newcommand{\rmB}{\mathrm B} \newcommand{\rmC}{\mathrm C} \newcommand{\rmD}{\mathrm D} \newcommand{\rmE}{\mathrm E} \newcommand{\rmF}{\mathrm F} \newcommand{\rmG}{\mathrm G} \newcommand{\rmH}{\mathrm H} \newcommand{\rmI}{\mathrm I} \newcommand{\rmJ}{\mathrm J} \newcommand{\rmK}{\mathrm K} \newcommand{\rmL}{\mathrm L} \newcommand{\rmM}{\mathrm M} \newcommand{\rmN}{\mathrm N} \newcommand{\rmO}{\mathrm O} \newcommand{\rmP}{\mathrm P} \newcommand{\rmQ}{\mathrm Q} \newcommand{\rmR}{\mathrm R} \newcommand{\rmS}{\mathrm S} \newcommand{\rmT}{\mathrm T} \newcommand{\rmU}{\mathrm U} \newcommand{\rmV}{\mathrm V} \newcommand{\rmW}{\mathrm W} \newcommand{\rmX}{\mathrm X} \newcommand{\rmY}{\mathrm Y} \newcommand{\rmZ}{\mathrm Z} \newcommand{\bb}{\mathbf{b}} \newcommand{\bv}{\mathbf{v}} \newcommand{\bw}{\mathbf{w}} \newcommand{\bx}{\mathbf{x}} \newcommand{\by}{\mathbf{y}} \newcommand{\bz}{\mathbf{z}} \newcommand{\paren}[1]{( #1 )} \newcommand{\Paren}[1]{\left( #1 \right)} \newcommand{\bigparen}[1]{\bigl( #1 \bigr)} \newcommand{\Bigparen}[1]{\Bigl( #1 \Bigr)} \newcommand{\biggparen}[1]{\biggl( #1 \biggr)} \newcommand{\Biggparen}[1]{\Biggl( #1 \Biggr)} \newcommand{\abs}[1]{\lvert #1 \rvert} \newcommand{\Abs}[1]{\left\lvert #1 \right\rvert} \newcommand{\bigabs}[1]{\bigl\lvert #1 \bigr\rvert} \newcommand{\Bigabs}[1]{\Bigl\lvert #1 \Bigr\rvert} \newcommand{\biggabs}[1]{\biggl\lvert #1 \biggr\rvert} \newcommand{\Biggabs}[1]{\Biggl\lvert #1 \Biggr\rvert} \newcommand{\card}[1]{\lvert #1 \rvert} \newcommand{\Card}[1]{\left\lvert #1 \right\rvert} \newcommand{\bigcard}[1]{\bigl\lvert #1 \bigr\rvert} \newcommand{\Bigcard}[1]{\Bigl\lvert #1 \Bigr\rvert} \newcommand{\biggcard}[1]{\biggl\lvert #1 \biggr\rvert} \newcommand{\Biggcard}[1]{\Biggl\lvert #1 \Biggr\rvert} \newcommand{\norm}[1]{\lVert #1 \rVert} \newcommand{\Norm}[1]{\left\lVert #1 \right\rVert} \newcommand{\bignorm}[1]{\bigl\lVert #1 \bigr\rVert} \newcommand{\Bignorm}[1]{\Bigl\lVert #1 \Bigr\rVert} \newcommand{\biggnorm}[1]{\biggl\lVert #1 \biggr\rVert} \newcommand{\Biggnorm}[1]{\Biggl\lVert #1 \Biggr\rVert} \newcommand{\iprod}[1]{\langle #1 \rangle} \newcommand{\Iprod}[1]{\left\langle #1 \right\rangle} \newcommand{\bigiprod}[1]{\bigl\langle #1 \bigr\rangle} \newcommand{\Bigiprod}[1]{\Bigl\langle #1 \Bigr\rangle} \newcommand{\biggiprod}[1]{\biggl\langle #1 \biggr\rangle} \newcommand{\Biggiprod}[1]{\Biggl\langle #1 \Biggr\rangle} \newcommand{\set}[1]{\lbrace #1 \rbrace} \newcommand{\Set}[1]{\left\lbrace #1 \right\rbrace} \newcommand{\bigset}[1]{\bigl\lbrace #1 \bigr\rbrace} \newcommand{\Bigset}[1]{\Bigl\lbrace #1 \Bigr\rbrace} \newcommand{\biggset}[1]{\biggl\lbrace #1 \biggr\rbrace} \newcommand{\Biggset}[1]{\Biggl\lbrace #1 \Biggr\rbrace} \newcommand{\bracket}[1]{\lbrack #1 \rbrack} \newcommand{\Bracket}[1]{\left\lbrack #1 \right\rbrack} \newcommand{\bigbracket}[1]{\bigl\lbrack #1 \bigr\rbrack} \newcommand{\Bigbracket}[1]{\Bigl\lbrack #1 \Bigr\rbrack} \newcommand{\biggbracket}[1]{\biggl\lbrack #1 \biggr\rbrack} \newcommand{\Biggbracket}[1]{\Biggl\lbrack #1 \Biggr\rbrack} \newcommand{\ucorner}[1]{\ulcorner #1 \urcorner} \newcommand{\Ucorner}[1]{\left\ulcorner #1 \right\urcorner} \newcommand{\bigucorner}[1]{\bigl\ulcorner #1 \bigr\urcorner} \newcommand{\Bigucorner}[1]{\Bigl\ulcorner #1 \Bigr\urcorner} \newcommand{\biggucorner}[1]{\biggl\ulcorner #1 \biggr\urcorner} \newcommand{\Biggucorner}[1]{\Biggl\ulcorner #1 \Biggr\urcorner} \newcommand{\ceil}[1]{\lceil #1 \rceil} \newcommand{\Ceil}[1]{\left\lceil #1 \right\rceil} \newcommand{\bigceil}[1]{\bigl\lceil #1 \bigr\rceil} \newcommand{\Bigceil}[1]{\Bigl\lceil #1 \Bigr\rceil} \newcommand{\biggceil}[1]{\biggl\lceil #1 \biggr\rceil} \newcommand{\Biggceil}[1]{\Biggl\lceil #1 \Biggr\rceil} \newcommand{\floor}[1]{\lfloor #1 \rfloor} \newcommand{\Floor}[1]{\left\lfloor #1 \right\rfloor} \newcommand{\bigfloor}[1]{\bigl\lfloor #1 \bigr\rfloor} \newcommand{\Bigfloor}[1]{\Bigl\lfloor #1 \Bigr\rfloor} \newcommand{\biggfloor}[1]{\biggl\lfloor #1 \biggr\rfloor} \newcommand{\Biggfloor}[1]{\Biggl\lfloor #1 \Biggr\rfloor} \newcommand{\lcorner}[1]{\llcorner #1 \lrcorner} \newcommand{\Lcorner}[1]{\left\llcorner #1 \right\lrcorner} \newcommand{\biglcorner}[1]{\bigl\llcorner #1 \bigr\lrcorner} \newcommand{\Biglcorner}[1]{\Bigl\llcorner #1 \Bigr\lrcorner} \newcommand{\bigglcorner}[1]{\biggl\llcorner #1 \biggr\lrcorner} \newcommand{\Bigglcorner}[1]{\Biggl\llcorner #1 \Biggr\lrcorner} \newcommand{\e}{\varepsilon} \newcommand{\eps}{\varepsilon} \newcommand{\from}{\colon} \newcommand{\super}[2]{#1^{(#2)}} \newcommand{\varsuper}[2]{#1^{\scriptscriptstyle (#2)}} \newcommand{\tensor}{\otimes} \newcommand{\eset}{\emptyset} \newcommand{\sse}{\subseteq} \newcommand{\sst}{\substack} \newcommand{\ot}{\otimes} \newcommand{\Esst}[1]{\bbE_{\substack{#1}}} \newcommand{\vbig}{\vphantom{\bigoplus}} \newcommand{\seteq}{\mathrel{\mathop:}=} \newcommand{\defeq}{\stackrel{\mathrm{def}}=} \newcommand{\Mid}{\mathrel{}\middle|\mathrel{}} \newcommand{\Ind}{\mathbf 1} \newcommand{\bits}{\{0,1\}} \newcommand{\sbits}{\{\pm 1\}} \newcommand{\R}{\mathbb R} \newcommand{\Rnn}{\R_{\ge 0}} \newcommand{\N}{\mathbb N} \newcommand{\Z}{\mathbb Z} \newcommand{\Q}{\mathbb Q} \newcommand{\mper}{\,.} \newcommand{\mcom}{\,,} \DeclareMathOperator{\Id}{Id} \DeclareMathOperator{\cone}{cone} \DeclareMathOperator{\vol}{vol} \DeclareMathOperator{\val}{val} \DeclareMathOperator{\opt}{opt} \DeclareMathOperator{\Opt}{Opt} \DeclareMathOperator{\Val}{Val} \DeclareMathOperator{\LP}{LP} \DeclareMathOperator{\SDP}{SDP} \DeclareMathOperator{\Tr}{Tr} \DeclareMathOperator{\Inf}{Inf} \DeclareMathOperator{\size}{size} \DeclareMathOperator{\poly}{poly} \DeclareMathOperator{\polylog}{polylog} \DeclareMathOperator{\min}{min} \DeclareMathOperator{\max}{max} \DeclareMathOperator{\argmax}{arg\,max} \DeclareMathOperator{\argmin}{arg\,min} \DeclareMathOperator{\qpoly}{qpoly} \DeclareMathOperator{\qqpoly}{qqpoly} \DeclareMathOperator{\conv}{conv} \DeclareMathOperator{\Conv}{Conv} \DeclareMathOperator{\supp}{supp} \DeclareMathOperator{\sign}{sign} \DeclareMathOperator{\perm}{perm} \DeclareMathOperator{\mspan}{span} \DeclareMathOperator{\mrank}{rank} \DeclareMathOperator{\E}{\mathbb E} \DeclareMathOperator{\pE}{\tilde{\mathbb E}} \DeclareMathOperator{\Pr}{\mathbb P} \DeclareMathOperator{\Span}{Span} \DeclareMathOperator{\Cone}{Cone} \DeclareMathOperator{\junta}{junta} \DeclareMathOperator{\NSS}{NSS} \DeclareMathOperator{\SA}{SA} \DeclareMathOperator{\SOS}{SOS} \DeclareMathOperator{\Stab}{\mathbf Stab} \DeclareMathOperator{\Det}{\textbf{Det}} \DeclareMathOperator{\Perm}{\textbf{Perm}} \DeclareMathOperator{\Sym}{\textbf{Sym}} \DeclareMathOperator{\Pow}{\textbf{Pow}} \newcommand{\iprod}[1]{\langle #1 \rangle} \newcommand{\cE}{\mathcal{E}} \newcommand{\E}{\mathbb{E}} \newcommand{\pE}{\tilde{\mathbb{E}}} \newcommand{\N}{\mathbb{N}} \renewcommand{\P}{\mathcal{P}} \notag $
$ \newcommand{\sleq}{\ensuremath{\preceq}} \newcommand{\sgeq}{\ensuremath{\succeq}} \newcommand{\diag}{\ensuremath{\mathrm{diag}}} \newcommand{\support}{\ensuremath{\mathrm{support}}} \newcommand{\zo}{\ensuremath{\{0,1\}}} \newcommand{\pmo}{\ensuremath{\{\pm 1\}}} \newcommand{\uppersos}{\ensuremath{\overline{\mathrm{sos}}}} \newcommand{\lambdamax}{\ensuremath{\lambda_{\mathrm{max}}}} \newcommand{\rank}{\ensuremath{\mathrm{rank}}} \newcommand{\Mslow}{\ensuremath{M_{\mathrm{slow}}}} \newcommand{\Mfast}{\ensuremath{M_{\mathrm{fast}}}} \newcommand{\Mdiag}{\ensuremath{M_{\mathrm{diag}}}} \newcommand{\Mcross}{\ensuremath{M_{\mathrm{cross}}}} \newcommand{\eqdef}{\ensuremath{ =^{def}}} \newcommand{\threshold}{\ensuremath{\mathrm{threshold}}} \newcommand{\vbls}{\ensuremath{\mathrm{vbls}}} \newcommand{\cons}{\ensuremath{\mathrm{cons}}} \newcommand{\edges}{\ensuremath{\mathrm{edges}}} \newcommand{\cl}{\ensuremath{\mathrm{cl}}} \newcommand{\xor}{\ensuremath{\oplus}} \newcommand{\1}{\ensuremath{\mathrm{1}}} \notag $
$ \newcommand{\transpose}[1]{\ensuremath{#1{}^{\mkern-2mu\intercal}}} \newcommand{\dyad}[1]{\ensuremath{#1#1{}^{\mkern-2mu\intercal}}} \newcommand{\nchoose}[1]{\ensuremath} \newcommand{\generated}[1]{\ensuremath{\langle #1 \rangle}} \notag $
$ \newcommand{\eqdef}{\mathbin{\stackrel{\rm def}{=}}} \newcommand{\R} % real numbers \newcommand{\N}} % natural numbers \newcommand{\Z} % integers \newcommand{\F} % a field \newcommand{\Q} % the rationals \newcommand{\C}{\mathbb{C}} % the complexes \newcommand{\poly}} \newcommand{\polylog}} \newcommand{\loglog}}} \newcommand{\zo}{\{0,1\}} \newcommand{\suchthat} \newcommand{\pr}[1]{\Pr\left[#1\right]} \newcommand{\deffont}{\em} \newcommand{\getsr}{\mathbin{\stackrel{\mbox{\tiny R}}{\gets}}} \newcommand{\Exp}{\mathop{\mathrm E}\displaylimits} % expectation \newcommand{\Var}{\mathop{\mathrm Var}\displaylimits} % variance \newcommand{\xor}{\oplus} \newcommand{\GF}{\mathrm{GF}} \newcommand{\eps}{\varepsilon} \notag $
$ \newcommand{\class}[1]{\mathbf{#1}} \newcommand{\coclass}[1]{\mathbf{co\mbox{-}#1}} % and their complements \newcommand{\BPP}{\class{BPP}} \newcommand{\NP}{\class{NP}} \newcommand{\RP}{\class{RP}} \newcommand{\coRP}{\coclass{RP}} \newcommand{\ZPP}{\class{ZPP}} \newcommand{\BQP}{\class{BQP}} \newcommand{\FP}{\class{FP}} \newcommand{\QP}{\class{QuasiP}} \newcommand{\VF}{\class{VF}} \newcommand{\VBP}{\class{VBP}} \newcommand{\VP}{\class{VP}} \newcommand{\VNP}{\class{VNP}} \newcommand{\RNC}{\class{RNC}} \newcommand{\RL}{\class{RL}} \newcommand{\BPL}{\class{BPL}} \newcommand{\coRL}{\coclass{RL}} \newcommand{\IP}{\class{IP}} \newcommand{\AM}{\class{AM}} \newcommand{\MA}{\class{MA}} \newcommand{\SBP}{\class{SBP}} \newcommand{\coAM}{\class{coAM}} \newcommand{\coMA}{\class{coMA}} \renewcommand{\P}{\class{P}} \newcommand\prBPP{\class{prBPP}} \newcommand\prRP{\class{prRP}} \newcommand\prP{\class{prP}} \newcommand{\Ppoly}{\class{P/poly}} \newcommand{\NPpoly}{\class{NP/poly}} \newcommand{\coNPpoly}{\class{coNP/poly}} \newcommand{\DTIME}{\class{DTIME}} \newcommand{\TIME}{\class{TIME}} \newcommand{\SIZE}{\class{SIZE}} \newcommand{\SPACE}{\class{SPACE}} \newcommand{\ETIME}{\class{E}} \newcommand{\BPTIME}{\class{BPTIME}} \newcommand{\RPTIME}{\class{RPTIME}} \newcommand{\ZPTIME}{\class{ZPTIME}} \newcommand{\EXP}{\class{EXP}} \newcommand{\ZPEXP}{\class{ZPEXP}} \newcommand{\RPEXP}{\class{RPEXP}} \newcommand{\BPEXP}{\class{BPEXP}} \newcommand{\SUBEXP}{\class{SUBEXP}} \newcommand{\NTIME}{\class{NTIME}} \newcommand{\NL}{\class{NL}} \renewcommand{\L}{\class{L}} \newcommand{\NQP}{\class{NQP}} \newcommand{\NEXP}{\class{NEXP}} \newcommand{\coNEXP}{\coclass{NEXP}} \newcommand{\NPSPACE}{\class{NPSPACE}} \newcommand{\PSPACE}{\class{PSPACE}} \newcommand{\NSPACE}{\class{NSPACE}} \newcommand{\coNSPACE}{\coclass{NSPACE}} \newcommand{\coL}{\coclass{L}} \newcommand{\coP}{\coclass{P}} \newcommand{\coNP}{\coclass{NP}} \newcommand{\coNL}{\coclass{NL}} \newcommand{\coNPSPACE}{\coclass{NPSPACE}} \newcommand{\APSPACE}{\class{APSPACE}} \newcommand{\LINSPACE}{\class{LINSPACE}} \newcommand{\qP}{\class{\tilde{P}}} \newcommand{\PH}{\class{PH}} \newcommand{\EXPSPACE}{\class{EXPSPACE}} \newcommand{\SigmaTIME}[1]{\class{\Sigma_{#1}TIME}} \newcommand{\PiTIME}[1]{\class{\Pi_{#1}TIME}} \newcommand{\SigmaP}[1]{\class{\Sigma_{#1}P}} \newcommand{\PiP}[1]{\class{\Pi_{#1}P}} \newcommand{\DeltaP}[1]{\class{\Delta_{#1}P}} \newcommand{\ATIME}{\class{ATIME}} \newcommand{\ASPACE}{\class{ASPACE}} \newcommand{\AP}{\class{AP}} \newcommand{\AL}{\class{AL}} \newcommand{\APSPACE}{\class{APSPACE}} \newcommand{\VNC}[1]{\class{VNC^{#1}}} \newcommand{\NC}[1]{\class{NC^{#1}}} \newcommand{\AC}[1]{\class{AC^{#1}}} \newcommand{\ACC}[1]{\class{ACC^{#1}}} \newcommand{\TC}[1]{\class{TC^{#1}}} \newcommand{\ShP}{\class{\# P}} \newcommand{\PaP}{\class{\oplus P}} \newcommand{\PCP}{\class{PCP}} \newcommand{\kMIP}[1]{\class{#1\mbox{-}MIP}} \newcommand{\MIP}{\class{MIP}} $
$ \newcommand{\textprob}[1]{\text{#1}} \newcommand{\mathprob}[1]{\textbf{#1}} \newcommand{\Satisfiability}{\textprob{Satisfiability}} \newcommand{\SAT}{\textprob{SAT}} \newcommand{\TSAT}{\textprob{3SAT}} \newcommand{\USAT}{\textprob{USAT}} \newcommand{\UNSAT}{\textprob{UNSAT}} \newcommand{\QPSAT}{\textprob{QPSAT}} \newcommand{\TQBF}{\textprob{TQBF}} \newcommand{\LinProg}{\textprob{Linear Programming}} \newcommand{\LP}{\mathprob{LP}} \newcommand{\Factor}{\textprob{Factoring}} \newcommand{\CircVal}{\textprob{Circuit Value}} \newcommand{\CVAL}{\mathprob{CVAL}} \newcommand{\CircSat}{\textprob{Circuit Satisfiability}} \newcommand{\CSAT}{\textprob{CSAT}} \newcommand{\CycleCovers}{\textprob{Cycle Covers}} \newcommand{\MonCircVal}{\textprob{Monotone Circuit Value}} \newcommand{\Reachability}{\textprob{Reachability}} \newcommand{\Unreachability}{\textprob{Unreachability}} \newcommand{\RCH}{\mathprob{RCH}} \newcommand{\BddHalt}{\textprob{Bounded Halting}} \newcommand{\BH}{\mathprob{BH}} \newcommand{\DiscreteLog}{\textprob{Discrete Log}} \newcommand{\REE}{\mathprob{REE}} \newcommand{\QBF}{\mathprob{QBF}} \newcommand{\MCSP}{\mathprob{MCSP}} \newcommand{\GGEO}{\mathprob{GGEO}} \newcommand{\CKTMIN}{\mathprob{CKT-MIN}} \newcommand{\MINCKT}{\mathprob{MIN-CKT}} \newcommand{\IdentityTest}{\textprob{Identity Testing}} \newcommand{\Majority}{\textprob{Majority}} \newcommand{\CountIndSets}{\textprob{\#Independent Sets}} \newcommand{\Parity}{\textprob{Parity}} \newcommand{\Clique}{\textprob{Clique}} \newcommand{\CountCycles}{\textprob{#Cycles}} \newcommand{\CountPerfMatchings}{\textprob{\#Perfect Matchings}} \newcommand{\CountMatchings}{\textprob{\#Matchings}} \newcommand{\CountMatch}{\mathprob{\#Matchings}} \newcommand{\ECSAT}{\mathprob{E#SAT}} \newcommand{\ShSAT}{\mathprob{#SAT}} \newcommand{\ShTSAT}{\mathprob{#3SAT}} \newcommand{\HamCycle}{\textprob{Hamiltonian Cycle}} \newcommand{\Permanent}{\textprob{Permanent}} \newcommand{\ModPermanent}{\textprob{Modular Permanent}} \newcommand{\GraphNoniso}{\textprob{Graph Nonisomorphism}} \newcommand{\GI}{\mathprob{GI}} \newcommand{\GNI}{\mathprob{GNI}} \newcommand{\GraphIso}{\textprob{Graph Isomorphism}} \newcommand{\QuantBoolForm}{\textprob{Quantified Boolean Formulae}} \newcommand{\GenGeography}{\textprob{Generalized Geography}} \newcommand{\MAXTSAT}{\mathprob{Max3SAT}} \newcommand{\GapMaxTSAT}{\mathprob{GapMax3SAT}} \newcommand{\ELIN}{\mathprob{E3LIN2}} \newcommand{\CSP}{\mathprob{CSP}} \newcommand{\Lin}{\mathprob{Lin}} \newcommand{\ONE}{\mathbf{ONE}} \newcommand{\ZERO}{\mathbf{ZERO}} \newcommand{\yes} \newcommand{\no} $
Back to blog

Sensitivity Conjecture

上禮拜整個理論CS界被Hao Huangㄧ篇只有六頁的論文震驚了:高懸三十年的Sensitivity Conjecture被解決了!

由於已經有眾多的blog bost(e.g., Scott Aaronson, Gil Kalai, Boaz Barak)介紹這個證明的美妙,在這邊我打算換個角度切入,來講講為什麼Sensitivity Conjecture是個有意思的東西。

一切可以從query complexity說起。對於任何一個boolean function $f:\bit^n\rightarrow\bit$,其中我們用$x\in\bit^n$來表示input variabes。一個很自然的complexity measure會是:在最壞的情況之下,需要query幾個input bits就足夠知道$f(x)$的值?舉例來說,如果$f$是一個constant function,那麼什麼input bit都不需要query我們就可以知道$f(x)$是多少了。而如果$f(x)=x_1\oplus x_1\oplus\cdots\oplus x_n$是個parity function,那麼可以看出來我們必須query所有的input bits。

上述的這個complexity measure,有個正式的名稱,叫做sensitivity,我們使用$s(f)$來表示。有意思的地方是,$s(f)$很自然地會是決策樹複雜度(decision tree complexity)的一個下界!回想一下,一個boolean function的決策樹是一個binary tree,其中每個內部的節點被一個input bit標記(e.g., $x_i$),相對應的兩個邊分別是0和1,而最底下的葉子則是被0和1標註。下圖是個3個input bits的parity function的一顆決策樹。

對於決策樹來說,我們在意的複雜度是它的深度,以下用$DDT(f)$來表示$f$最淺的決策樹的深度。而不難看出來,如果$f$有個深度為$d$的決策樹,那麼$f$的sensitivity最多為$d$,因為對於任何一個input $x$來說,最多query $d$個bits就可以走到決策樹的底端了。

For any $n\in\N$ and $f:\bit^n\rightarrow\bit$, $s(f)\leq DDT(f)$.

有了上述這個定理,一個很自然的問題即是,另一個方向會不會也成立呢?也就是說,$DDT(f)$可不可以被$s(f)$有效率的upper bound呢?比較正式的問法則是,sensitivity和決測樹複雜度是不是多項式相關的(polynomially related)。這個問題看似很單純簡單的問題,其實是sensitivity conjecture的一個等價的陳述!

Does there exist a polynomial $p$ such that for any $n\in\N$ and $f:\bit^n\rightarrow\bit$, $DDT(f)\leq p(s(f))$?

如果sensitivity conjecture的故事到這邊就結束了,那麼實在是看不出來他有去的地方在哪裡,充其量就是個關於兩種複雜度之間根本關係的一個問題。有趣的地方來了,除了這兩種query complexity的複雜度之外,人們還定義了十幾種的query complexity,而且除了sensitivity之外,其他任意兩種複雜度他們之間都是多項式相關的!

這樣講可能還是很虛幻,讓我定義一個常見的quary complexity複雜度,給各位一點點感覺。這個複雜度是block sensitivity,和sensitivity乍看之下非常地像。對於boolean function$f$還有input $x$來說,相對應的block sensitivity定義如下:令$B\subseteq[n]$,如果$f(x)\neq f(x^{(B)})$,那我們則稱$B$對於$f$和$x$來說很敏感。這邊$x^{(B)}$的意思是把$x$中在$B$相對應位置的bit都翻轉。而$f$在$x$上的block sensitivity則定義為最大的$k\in\N$使得存在$B_1,B_2,\dots,B_k\subseteq[n]$兩兩不相交,且每個$B_i$都對$f$和$x$來說很敏感!於是$f$本身的block sensitivity也就很自然的定義為所有input $x$中最大的block sensitivity,我們用$bs(f)$來表示。以下是一個簡單的例子。

令$m$為一個偶數,考慮一個$m\times m$的grid,令$n=m^2$,並且讓每個grid point上面放一個變數。定義Rubinstein function $f$如下:$f(x)=1$當存在一個row裡面恰好有兩個連續點被設為$1$。 Rubinstein function的block sensitivity是$n/2$,因為對於全部是$0$的input來說,兩個位於同一個row且相鄰格形成了一個sensitive的block。然而不難看出,Rubinstein function的sensitivity只有$m=\sqrt{n}$。

現在有了block sensitivity的概念後,上述的sensitivity conjecture可以被等價地陳述如下。

Does there exist a polynomial $p$ such that for any $n\in\N$ and $f:\bit^n\rightarrow\bit$, $bs(f)\leq p(s(f))$?

除了block sensitivity之外,如剛才所說,還有好多有趣的複雜度彼此之間都是多項式相關(有興趣的讀者可以參考這個survey),就是除了跟sensitivity之外。而現在他們終於全部都互相多項式相關了,thanks to Huang Hao!