Garrett: Integral moments [Edinburgh, 04 Aug 2008]
Integral moments Paul Garrett
[email protected] http://www.math.umn.edu/˜garrett/ (with Diaconu and Goldfeld)
• Idea: integral moments over families of L-functions arise as coefficients in automorphic spectral decompositions. • Example: in several cases, extract subconvex bounds • Note: in some applications a subconvex bound replaces Lindel¨of Sample of moments and bounds: Euler-Riemann zeta tε ∀ε>0 (unknown) of ζ( 21 + it) ≪ (RH ⇒)Lindel¨
1
convexity
2k th
∀ε>0 (true, easy) ζ( 21 + it) ≪ t 4 +ε Z T |ζ( 21 + it)|2k dt moment = 0
Example of integral moments Z ∞ |ζ( 12 + it)|2k t−w dt Zk (w) = 1
which has natural boundary for k ≥ 3 (Diaconu-PG-Goldfeld). Z T |ζ( 12 + it)|2k dt ≪ T 1+ε Lindel¨of ⇐⇒ (for all k, ε > 0) 0
Suitable moment estimates yield subconvex bounds: RT |ζ( 21 + it)|2 dt ≪ T 1+ε 6 ⇒ subconvex bd = 0 RT 6 ⇒ subconvex bd = |ζ( 12 + it)|4 dt ≪ T 1+ε 0 RT |ζ( 21 + it)|4 dt = T P (log T ) + O(T 1−δ ) 0 RT |ζ( 12 + it)|2k dt ≪ T 1+ε with 2k ≥ 6 0 1
=⇒ subconvex bd
=⇒ subconvex bd
Garrett: Integral moments [Edinburgh, 04 Aug 2008] 1918 Hardy-Littlewood:
Z
T 0
|ζ( 21 + it)|2 dt ∼ T log T (6=⇒ subconvex bound) 1
1921 Weyl (not by moments): |ζ( 12 + it)| ≪ (1 + |t|) 6 +ε (= subconvex) Z T |ζ( 12 + it)|4 dt ∼ 2π1 2 T (log T )4 (6=⇒ subconvex bound) 1926 Ingham: 0
1979 Heath-Brown: Z T 7 |ζ( 12 + it)|4 dt ∼ T · P (log T ) + O(T 8 +ε ) (=⇒ subconvex bound) 0
Our prototype: 1982, A. Good: holo cuspform f Z T |L( 12 + it, f )|2 dt = aT (log T +b) + O((T log T )2/3 ) =⇒ subconvex 0
Proof mechanism: (1) automorphic spectral decomposition (2) L-functions as decomposition coefficients. Examples of decomposition coefficients and L-functions: Z 1 y s− y 2f = Λ(s, f ) (f on GL2 ) 1
f g , Es = Λ(s, f ⊗ g) (f, g on GL2 )
Z
Decomposition example: SL2 ( )\H Z X hΦ, F i hΦ, 1i 1 Φ ∼ hΦ, Es i · Es ds + ·F + ·1 hF, F i 4πi Re (s)= 21 h1, 1i F cfm
Over a number field: Z X hΦ, F i 1 X Φ ∼ hΦ, Es,χ i · Es,χ ds ·F + hF, F i 4πiκ χ Re (s)= 12 F cfm
X hΦ, χ ◦ deti · (χ ◦ det) + ||χ ◦ det || 2 χ =1
See sum over χ, integral over t. 2
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Create integral kernel/Poincar´e series Pα,w such that: Integral moment of cuspform f produced by integral Z Z 1 α,w 2 L( 12 + it + α, f ) · L( 12 − it, f ) · ∼ t−w dt P · |f | = 2π Spectral expansion of Pα,w α,w
=
P
π
1−w 2
Γ( w−1 1 X L( 21 + α, F ) 2 ) · E1+α + · G( 12 − itF , α, w) · F w −w 2 2 hF, F i π Γ( 2 ) F on GL 2
+
1 4πi
Z
Re (s)= 21
ζ(α + s) ζ(α + 1 − s) G(1 − s, α, w) · Es ds ξ(2 − 2s)
with G(s, α, w) = π
Γ( −(α+ w 2 )
α+1−s α−s+w ) Γ( α+s ) Γ( α+s−1+w ) 2 2 ) Γ( 2 2 Γ(α + w2 )
Continuous part cancels pole of leading term at α = 0. Evaluated at α = 0: Z Z 2 1 1 0,w 2 P · |f | = L( 2 + it, f ) · ∼ t−w dt 2π and
0,w
P
1 = pole at w = 1 + 2 1 + 4πi
Over
Z
Re (s)= 21
X
F on GL2
L( 12 , F ) · G( 21 − itF , 0, w) · F hF, F i
w−1+s ) ξ(s) ξ(1 − s) Γ( w−s 2 ) Γ( 2 · Es ds ξ(2 − 2s) Γ( w2 )
Q(i) grossencharacters appear (Diaconu-Goldfeld 2006)
Similarly over number fields. With suitable data, Theorem: t-aspect subconvexity for GL2 over number fields (Diaconu-PG 2006) 3
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Proof ingredients: First, over
Q:
Spectral expansion of Pα,w gives meromorphic continuation Obtain meromorphic continuation of generating function Z(w) Leading pole at w = 1. Trailing poles at µ = µf with µ(µ − 1) eigenvalue for waveform f . Need polynomial vertical growth Need spectral gap (Kim-Shahidi): separate cuspidal poles from leading pole Need asymptotics of triple integrals Z F · |f |2 of eigenfunctions (Sarnak, Bernstein-Reznikoff, Kr¨ otz-Stanton). exponentially decreasing, not merely rapidly.
Namely,
See also Goldfeld-Hoffstein-Lockhart, Hoffstein-Ramakrishnan for asymptotics of L( 12 , F ) (F cuspform) hF, F i Use positivity of moment sum-and-integral ... Landau’s lemma
Complications over number field: Freeze parameters w at all but one place... thus, breaking t-aspect convexity at a single place, not hybrid Poles at eigenvalues of one Laplacian presumably accumulate, ... requiring finesse proving polynomial vertical growth
4
Garrett: Integral moments [Edinburgh, 04 Aug 2008] The diagonal/positivity property: The diagonal form
XZ
|L( 21 + it, f ⊗ χ)|2 . . .
χ
rather than a smeared-out form X Z Z L( 21 + it1 , f ⊗ χ1 ) · L( 12 − it2 , f ⊗ χ2 ) . . . χ1 ,χ2
is essential, or at least extremely convenient. Arises as deformation of diagonal distribution. Simpler example: Distribution u on S 1 × S 1 integrating along the diagonal Z f ·g u(f ⊗ g) = S1
Has diagonal Fourier expansion u =
X
e2πinx ⊗ e−2πiny
n
since, by Plancherel, Z X X f ·g = fb(n)b g (−n) = fd ⊗ g(m, n) · u b(m, n) S1
n
m,n
Similarly for any diagonal integral. Clean-but-harsh pure diagonal distributions often usefully deformed to nearlydiagonal more-classical function.
5
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Something appealing that doesn’t work: to obtain 2k th moments for GL2 cuspforms? Corresponding idea in more tangible situation of classical Fourier series: let u be the distribution on H = S1 × . . . × S1 {z } | 2k
given by integration along the subgroup
Θ = {(x1 , . . . , xk , y1 , . . . , yk ) :
x1 . . . xk = 1} ⊂ H y1 . . . yk
The Fourier expansion is concentrated along a diagonal line: X u = e2πin(x1 +...+xk −y1 −...−yk ) n
For automorphic forms: let n x1 xk y1 H = ×...× × 1 1
1
×...×
yk 1
o
≈ GL2k 1
and let u be integration along o n x1 . . . xk = 1 Θ = elements in H with : y1 . . . yk For cuspform f on GL2 , restrict F = f ⊗ ... ⊗ f ⊗ f ⊗ ...f | {z } 2k
Q
to Hk \HA and evaluate u on it. Over , produces Z ∞ 2k 1 Λ( 2 + it, f ) dt −∞
Obstacle: spectral interpretation/expansion for 2k > 2? Can deform to soften exponential decay of gamma factors, but... 6
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Technicality: the weight in the integral moment At α = 0, finite-prime factors are literal Hecke-Jacquet-Langlands Mellin transforms of local Whittaker functions Z 1 y |y|s− 2 χ(y) Wf,v Lv (s, f ⊗ χ) = × 1 kv and Z Z 2 y ′ it 1 |y/y | · Wf,v Lv ( 2 + it, f ) =
1
W f,v
y′ 1
Deformation at archimedean places entangles this with integration over unipotent radical: with additive character ψ and ′ 1 x y y ′ n= h= h = 1 1 1 |Lv ( 21 + it, f )|2 is deformed into local factor at archimedean v = =
Z Z Z
Z Z Z =
s− 12
ϕv (n) |y ′ /y|v
· Wf,v (hn) · W f,v (h′ n)·
s− 1 ϕv (n) ψ (y − y ′ )x · Wf,v (h) · W f,v (h′ ) · |y ′ /y|v 2
Z Z
s− 21
ϕ bv (y − y ′ ) · Wf,v (h) · W f,v (h′ ) · |y ′ /y|v
Except for holomorphic discrete series, these seem not usefully expressible by classical special functions. The map from data ϕv to these integrals is definitely not surjective to an elementary space of functions. 1 x Nevertheless, asymptotically t−w for data ϕv = (1 + x2 )−w/2 1 7
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Other features/possibilities for GL2 Over quadratic extensions of same proof.
Q, should break convexity in χ-aspect, with
Full χ-aspect over number field subtler, like hybrid bounds.
Allow deformation at finite place, should break convexity in depth: unlimited ramification of χ’s at fixed place. (Delia Letang, work-in-progress)
Replace cuspform by (packet of) Eisenstein series: ... should break t-aspect convexity for Dedekind zetas of number fields, via fourth moments. ... and should break t-aspect convexity for Hecke L-functions with grossencharacters, via fourth moments. ... and should break χ-aspect convexity for quadratic fields. ... and should break convexity in depth by deforming finite prime data.
8
Garrett: Integral moments [Edinburgh, 04 Aug 2008] Higher rank example
(Diaconu-PG-Goldfeld, 2006)
Q
Create Pα,w to produce moment expansion for GL3 ( ) cuspform f Z
0,w
P
2
· |f |
=
X
F on GL2
Z
|L( 12 + it, f ⊗ F )|2 MF dt hF, F i
Z Z 1 L( 2 + it1 − it2 , f ) · L( 21 + it1 + it2 , f ) 2 ME dt1 dt2 + ζ(1 − 2it2 )
More-continuous part is sort of integral moment of standard L(s, f ) of f . Spectral expansion of GL3 Poincar´e series is induced from GL2 Pα,w +
X
F on GL2
+
Z
Re (s)= 12
=
2,1 (∞−part) · Eα+1
L( 3α+1 + 12 , F ) 2 (∞−part) · · E 1,2 α+1 hF, F i 2 ,F
+ 1 − s) · ζ( 3α+1 + s) ζ( 3α+1 2 2 (∞−part) · ζ(2 − 2s) 1,1,1 × Eα+1, ds s− α+1 , −s− α+1 2
2
Only GL2 cuspforms appear in spectral expansion of P! No GL3 cuspforms! It is good that no cuspidal data beyond GL2 appears. But this is also confusing: obscures easiest heuristics for computing and understanding spectral expansion of P. In fact, Pα,w is a residue of overlying Poincar´ e series Qα,β,w with more balanced spectral expansion, and retaining meaningful moment expansion. More on this later.
9
Garrett: Integral moments [Edinburgh, 04 Aug 2008]
Q
GLn ( ) more generally Moment expansion: Z P0,w · |f |2 =
X
F on GLn−1
Z
Re (s)= 21
|L(s, f ⊗ F )|2 MF (s) ds + . . . hF, F i
Spectral expansion of Pα,w Pα,w +
X
F on GL2
+
=
n−1,1 (∞−part) · E1+α
L( nα+n−2 + 21 , F ) 2 (∞−part) · · E n−2,2 1+α hF, F i 2 ,F
Z
L( nα+n−2 + 1 − s, χ) · L( nα+n−2 + s, χ) 2 2 (∞−part) × L(2 − 2s, χ2 ) Re (s)= 12 n−2,1,1 × Eα+1, s−(n−2) α+1 , −s−(n−2) α+1 2
ds
2
Only GL2 cuspforms appear in spectral expansion of P! No GL3 , GL4 , GL5 , . . . cuspforms! Construction of kernel Pα,w 1n−1 ∗ U= 0 1
(n − 1) 0 H= 0 1 N Z = center G, Kv maximal compact in Gv . Let ϕ = v ϕv with α A 0 ϕv ( · Kv ) = (det A)/dn−1 v (for v finite) 0 d
Extend by 0 off Hv Zv Kv . For v|∞ require the same right left equivariance and Kv -invariance, with ϕv determined by values on Uv . For example, take 1n−1 x ϕv = (1 + |x1 |2 + . . . + |xn−1 |2 )−w/2 0 1 Kernel is α,w
P
(g) =
X
γ∈Zk Hk \Gk
10
ϕ(γg)
Garrett: Integral moments [Edinburgh, 04 Aug 2008] More-continuous terms: higher integral moments Most-continuous part of moment expansion for GLn Z Z )|2 M dt de t |L( 12 + it, f ⊗ E min 1 +ie t 2
Λ
where
Z Z Π1≤ℓ≤n−1 L( 12 + it + itℓ , f ) 2 e = Π1≤j
