Bernstein s analytic continuation of complex powers

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1 (April 3, 20) Bernstein s analytic continuation of complex powers Paul Garrett garrett@math.umn.edu garrett/. Analytic continuation of distributions 2. Statement of the theorems on analytic continuation 3. Bernstein s proofs 4. Proof of the Lemma: the Bernstein polynomial 5. Proof of the Proposition: estimates on zeros Let f be a polynomial in x,..., x n with real coefficients. For complex s, let f s + be the function defined by f s +(x) = f(x) s (for f(x) 0) 0 (for f(x) 0) Certainly for Re(s) 0 the function f+ s is locally integrable. For s in this range, define a distribution, also denoted f+, s by f+(φ) s = f+(x) s φ(x) dx (for φ Cc (R n )) R n The object is to analytically continue the distribution f s +, as a meromorphic (distribution-valued) function of s. One should also ask about analytic continuation as a tempered distribution. Several provocative examples appear in [Gelfand-Shilov 964]. In a lecture at the 963 Amsterdam Congress, I.M. Gelfand refined this question to require further that one show that the poles lie in a finite number of arithmetic progressions. [Bernstein 968] gave a first proof, under a regularity hypothesis on the zero-set of the polynomial f. We essentially reproduce that argument here, with supporting material from complex function theory and from the theory of distributions. [Bernstein-Gelfand 969] proved the meromorphic continuation without any hypotheses, but assuming desingularization [Hironaka 966]. [Atiyah 970] gave a similar argument. Although this resolved the original question, invocation of desingularization was unsatisfactory. [Bernstein 97] created the theory of D-modules, and [Bernstein 972] used it to prove the existence of the Bernstein polynomial F f (s) for any f, and so on. Invocation of desingularization was avoided, and much more was done, besides. Thanks to Bill Casselman for suggesting revisions, especially bibliographic, to an old version of this note.. Analytic continuation of distributions We recall the nature of the topologies on test functions and on distributions. Let Cc (U) be the collection of compactly-supported smooth functions with support inside a set U R n. As usual, for U compact, we have a countable family of seminorms on Cc (U): where for ν = (ν,..., ν n ), as usual, µ ν (f) = sup D ν f x D ν = ( x ) ν ( ) νn... x n

2 It is elementary that (for U compact) Cc (U) is a Fréchet space. To treat U not necessarily compact (e.g., R n itself) let U U 2... be compact subsets of U whose union is U. Then C c (U) is the union of the spaces C c (U i ), with the locally convex colimit (direct limit) topology. There is a countable cofinal sub-colimit, and each Fréchet space is closed in the next, so the colimit is called an LF-space (strict-(co-)limit-of-fréchet). LF-spaces are not complete metrizable, but are quasi-complete, meaning that bounded Cauchy nets converge. The spaces D (U) and D (R n ) of distributions on U and on R n, respectively, are the continuous duals of D(U) = C c (U) and D(R n ) = C c (R n ). The topology on the continuous dual space V of a topological vectorspace V is the weak- topology: a sub-basis near 0 in V consists of sets U v,ɛ = {λ V : λ(v) < ɛ} A V -valued function f on an open subset Ω of C is holomorphic on Ω when, for every v V, the C-valued function z f(z)(v) on Ω is holomorphic in the usual sense. More precisely, this is weak- holomorphy, referring to the topology. Let z o Ω for an open subset Ω of C and f a holomorphic V -valued function on Ω z o. Say that f is weakly meromorphic at z o when, for every v V, the C-valued function z f(z)(v) has a pole (as opposed to essential singularity) at z o. Say that f is strongly meromorphic at z o if the orders of these poles are bounded independently of v. That is, f is strongly meromorphic at z o if there is an integer n and an open set Ω containing z o so that, for all v V the C-valued function z (z z o ) n f(z)(v) is holomorphic on Ω. If n is the least integer f so that (z z o ) n f is holomorphic at z o, then f is of order n at z o, etc. To say that f is strongly meromorphic on an open set Ω is to require that there be a set S of points of Ω with no accumulation point in Ω so that f is holomorphic on Ω S, and so that f is strongly meromorphic at each point of S. For brevity, but risking some confusion, we will often say meromorphic rather than strongly meromorphic. 2. Statements of theorems on analytic continuation Let O be the polynomial ring R[x,..., x n ]. For z R n, let O z be the local ring at z, i.e., the ring of ratios P/Q of polynomials where the denominator does not vanish at z. Let m z be the maximal ideal of O z consisting of elements of O z whose numerator vanishes at z. Let I z (depending upon f) be the ideal in O z generated by,..., x x n A point z R n is said to be simple with respect to the polynomial f when f(z) = 0 I z m N z for some N There are α i m z so that f = i α i [2..] Remark: The second condition is equivalent to the O z /I z being finite-dimensional. The simplest situation in which the second condition holds is when I z = O z, i.e., some partial derivative of f is non-zero 2

3 at z. The third condition does not follow from the first two: Bernstein notes that the first two conditions hold but the third does not for f(x, y) = x 5 + y 5 + x 2 y 2 [2..2] Theorem: (local version) For z a simple point with respect to f, there is a neighborhood U of x so that the distribution f+,u s (φ) = f+(x) s φ(x) dx on test functions φ C c (U) on U has an analytic continuation to a meromorphic C c (U) -valued function. [2..3] Theorem: (global version) If all real zeros of f(x) are simple (with respect to f), then f s + is a meromorphic (distribution-valued) function of s C. 3. Bernstein s proof Let R z be the ring of linear differential operators with coefficients in O z. Note that R z is both a left and a right O-module: for D R z, for f, g O and φ a smooth function near z, the definition is (fdg)(φ) = f D(gφ) [3..] Lemma: There is a differential operator D R z and a non-zero Bernstein polynomial H in a single variable so that D(f n+ ) = H(n)f n for any natural number n. (Proof below.) Proof of Local Theorem from Lemma: Let U be a small-enough neighborhood of z so that on it all coefficients of D are holomorphic on U. For sufficiently large n the function f n+ + is continuously differentiable, so Df n+ + = H(n)f n + For each fixed φ C c (U) consider the function g(s) = (Df s+ + H(s)f s +)(φ) The hypotheses of the proposition below are satisfied, so the equality for all large-enough natural numbers implies equality everywhere: Df s+ + = H(s)f s + This gives f s + = Df s+ + H(s) Now claim that for any 0 n Z the distribution f+ s on Cc (U) is meromorphic for Re(s) > n. For n = 0 this is clear. The formula just derived gives the induction step. Further, this argument shows that the poles of f+ s restricted to Cc (U) are concentrated on the finite collection of arithmetic progressions λ i, λ i, λ i 2,... where the λ i are the roots of H(s). In particular, the order of the pole of f s + at a point s o is equal to the number of roots λ i so that s o lies among λ i, λ i, λ i 2,... 3

4 In particular, the distribution f s + really is (strongly) meromorphic. This proves the Theorem, granting the Lemma and granting the Proposition. /// [3..2] Proposition: (attributed by Bernstein to Carlson) For g analytic for Res > 0 and g(s) < be cre s and g(n) = 0 for all sufficiently large natural numbers n, we have g 0. Proof: (of Global Theorem) Invoking the Local Theorem and its proof above, for each z R n choose a neighborhood U z of z in which f s + is meromorphic, so that U z is Zariski-open, i.e., is the complement of a finite union of zero sets of polynomials. Indeed, writing f(x) = α i with α i O z, as in the proof of the Local Theorem, let α i = g i /h i with polynomials g i and h i, and take U z to be the complement of the union of the zero-sets of the denominators h i. By Hilbert s Basis Theorem, R n is covered by finitely-many U z,..., U zn. Make a partition of unity subordinate to this finite cover, i.e., take ψ,..., ψ n so that ψ i 0, ψ i, and sptψ i U zi. Then f s + = i ψ i f s + By choice of the U zi, the right-hand side is a finite sum of meromorphic (distribution-valued) functions. 4. Proof of the Lemma: the Bernstein polynomial Now we prove existence of the differential operator D and the Bernstein polynomial H. This is the most serious part of the argument. (The complex function theory proposition is not trivial, but is standard). Proof: (of Lemma) Let where the α i m z are so that Put Then By Leibniz formula, P (f) = i P = α i R z f = α i R z S i = P f = (P + ) Q α i = f S i f = f f = 0 P (f n ) = nf n S i (f n ) = 0 [4..] Sublemma: There is a non-zero polynomial M in one variable so that M(P ) = i J i (for some J i R z ) Proof: (of Sublemma) As usual, write ν = ν ν n. For a natural number m, move all the coefficients to the right of the differential operators, that is, write P m = D ν γ m,ν (where γ m,ν O z ) ν m 4

5 That this is possible is easy to see: x i 0 (for i j) x i = x j x j (for i = j) The coefficients γ m,ν are polynomials in the α i, so γ m,ν m ν z. Taking M(P ) of the form M(P ) = m q b m P m = m,ν D ν b m γ m,ν (with b m R) the condition of the sublemma will be met if b m γ m,ν I z (for all indices ν) m If ν N, where I m N, then this condition is automatically fulfilled. conditions b m γ m,ν = 0 m Thus, there are finitely-many where γ m,ν is the image of γ m,ν in O z /m N z. Since the latter quotient is, by hypothesis, a finite-dimensional vector space, the collection of such conditions gives a finite collection of homogeneous equations in the coefficients b m. More specifically, the number of such conditions is dim O z /m N z card{ν : ν < N} Taking q large enough assures the existence of a non-trivial solution {b m }, proving the sublemma. /// Returning to the proof of the lemma: as an equation in R z Put Then, as desired, M(P )(P + ) = J i (P + ) = J i S i + J i f D = J i H(P ) = M(P )(P + ) D(f n+ ) = ( J i f)(f n ) = H(P )(f n ) = H(n)f n This proves the Lemma, constructing the differential operator D. /// 5. Proof of the Proposition: estimates on zeros The necessary result is standard, although not so elementary as to be an immediate corollary of Cauchy s Theorem: [5..] Proposition: For g an analytic function for Res > 0 and g(s) < be cre s, if g(n) = 0 for all sufficiently large natural numbers n, then g 0. Proof: Consider G(z) = e c g( z + z ) 5

6 Then g is turned into a bounded function G on the disc, with zeros at points (n )/(n + ) for sufficiently large natural numbers n. We claim that, for a bounded function G on the unit disc with zeros ρ i, either G 0 or ( ρ i ) < + If we prove this, then in the situation at hand the natural numbers are mapped to i ρ n = (n )/(n + ) = n + so here ( ρ n ) = n n Thus, we would conclude G 0 as desired. n + = + Recall Jensen s formula: for any holomorphic function G on the unit disc with G(0) 0 and with zeros ρ,..., for 0 < r < we have G(0) ρ i r r ρ i { π } = exp log G(re iθ ) dθ 2π π Granting this, the assumed boundedness of G on the disc gives an absolute constant C so that for all N G(0) ρ i r (We can harmlessly divide by a suitable power of z to guarantee that G(0) 0.) Then, letting r, r ρ i C ρi G(0) C For an infinite product of positive real numbers ρ i less than to have a value > 0, it is elementary that we must have ( ρ i ) < + i as claimed. This proves the proposition. /// While we re here, let s recall the proof of Jensen s formula (e.g., as in [Rudin 966], for example. 0 < r < and let H(z) = G(z) r 2 ρz ρ r(ρ z) ρ z where the first product is over roots ρ with ρ < r and the second is over roots with ρ = r. Then H is holomorphic and non-zero in an open disk of radius r + ɛ for some ɛ > 0. Thus, log H is harmonic in this disk, and has the mean value property log H(0) = π log H(re iθ ) dθ 2π π Fix On one hand, H(0) = G(0) r ρ 6

7 On the other hand, if z = r the factors have absolute value. Thus, log H(re iθ ) = log G(re iθ ) As noted in Rudin (see below), ρ =r r 2 ρz r(ρ z) log e i(θ arg ρ) (where e i arg ρ = ρ) π log e iθ dθ = 0 2π π Therefore, the integral in the assertion of the mean value property is unchanged upon replacing H by G. Putting this all together gives Jensen s formula. /// We execute the integral computation, following Rudin. Since the disk is simply-connected, there is a function λ(z) on the open unit disc so that exp(λ(z)) = z We completely specify this λ by requiring λ(0) = 0. We have Reλ(z) = log z and Imλ(z) < π 2. Let δ > 0 be small. Let Γ = Γ δ be the (counterclockwise) path around the unit circle from e iδ to e (2π δ)i and let γ = γ δ be the (clockwise) path around a small circle centered at from e (2π δ)i to e iδ. Then 2π 2π log e iθ dθ = lim 0 2π [ = Re λ(z) dz ] = Re 2πi Γ z δ 0 2π δ δ [ 2πi log e iθ dθ γ λ(z) dz ] z by Cauchy s theorem. Elementary estimates show that the latter integral has a bound of the form Cδ log(/δ), which goes to 0 as δ 0. [Atiyah 970] M.F. Atiyah Resolution of singularities and division of distributions, Comm. Pure. Appl. Math. 23 (970), [Bernstein 968] I.N. Bernstein, The possibility of analytic continuation of f+ λ (Russian), Fun. An. and Applications 2 (968) no., for certain polynomials f [Bernstein-Gelfand 969] I.N. Bernstein and S.I. Gelfand, Meromorphy of the function P λ (Russian), Fun. An. and Applications 3 (969), no., [Bernstein 97] I.N. Bernstein, Modules over rings of differential operators; an investigation of the fundamental solutions of equations with constant coefficients (Russian), Fun. An. and Applications 5 (97) no. 2, -6. [Bernstein 972] I.N. Bernstein, Analytic continuation of generalized functions with respect to a parameter (Russian), Fun. An. and Applications 6 (972) no. 4, [Gelfand-Shilov 964] I.M. Gelfand, G.E. Shilov, Generalized functions, vol. I: properties and operations, Academic Press, 964 (Russian original, Fizmatgiz, 958). [Hironaka 964] H. Hironaka, Resolution of singularities of an algebraic variety over a field of characteristic zero, I, II, Ann. of Math. 79 (964), , [Rudin 966] W. Rudin, Real and Complex Analysis, McGraw-Hill,