# 1. Periodic Fourier series. The Fourier expansion of a 2π-periodic function f is:

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1 CONVERGENCE OF FOURIER SERIES 1. Periodic Fourier series. The Fourier expansion of a 2π-periodic function f is: with coefficients given by: a n = 1 π f(x) a a n cos(nx) + b n sin(nx), n 1 f(x) cos(nx)dx (n 0), b n = 1 π f(x) sin(nx)dx (n 1). The symbol above reminds us that this is a merely formal expansion- the formal series of functions on the right will diverge in many cases, and one needs to understand in what sense it approximates f. The Fourier sine and cosine series, that is, the representations of f as infinite series of eigenfunctions in [0, π] with Dirichlet (resp, Neumann) boundary conditions is a special case of this. It is very easy to see that a n vanishes if f is an odd function, while b n vanishes if f is even. Thus the analysis of Fourier cosine (resp. sine) series of a function f in [0, π] is equivalent to the analysis of the extension of f, first to [, π] as an even (resp. odd) function, then to all of R with period 2π. This remark also helps us choose a natural space of periodic functions to work with, namely, piecewise continuous functions. Much as one might prefer to work with continuous ones, the extension (as above) to R of a continuous function in [0, π] is, in general, only piecewise continuous (recall this means f is continuous at all but a finite number of points in [0, π], and at discontinuities the one-sided limits exist and are finite; in particular, f is bounded.) We adopt the notation C pw for 2π-periodic, piecewise continuous functions with piecewise-continuous first-derivatives. More generally, we say f C k per if it is 2π-periodic, with continuous derivatives up to order k and f (k+1) is piecewise continuous; f C per means f is 2π-periodic, continuous, and f is piecewise continuous. 2. Types of convergence. We are interested in the convergence of the sequence of partial sums: s N (x) = a 0 N 2 + a n cos nx + b n sin nx. Three types of convergence will be considered: 1

2 1. f n f pointwise on a set I if f n (x) f(x) for all x I; 2. f n f uniformly on a set I if: sup f n (x) f(x) 0. x I ( sup can be thought of as maximum value ) 3. f n f in L 2 norm if (f n f) 2 0. Pointwise convergence is fairly weak: for example, the pointwise limit of continuous functions is in general not continuous (example: f n (x) = x n in [0, 1]). Uniform convergence is the most natural concept (f is continuous if the f n are and convergence is uniform), but for this very reason is too much to hope for in many cases. L 2 convergence corresponds to approximation in an average sense, but it is not easy to see what it means. For one thing, if f n f in L 2, you can change the f n and f on a set of measure 0 (for example, on any finite set), and the statement will still be true: integrals are insensitive to what happens in sets of measure 0, so in particular if a sequence of functions has a limit in the L 2 sense, the limit is never unique! Horrible! You would have to declare two functions to be equal if they differ on a set of measure 0, but that opens a whole new can of worms. A closely related concept, but easier to grasp, is convergence in L 1 norm : f n f 0. Geometrically, this means the area of the region bounded by the graphs of f n and f (say, in [, π]) tends to zero. (And I mean the geometric area, not the signed area of Calculus; that s the effect of the absolute value in the definition.) Unfortunately, this interpretation does not hold for L 2 convergence. There are important logical relationships between these notions of convergence. For instance, uniform convergence easily implies pointwise convergence, L 2 convergence and L 1 convergence (this is really easy! Check it!). Also, consider the chain of inequalities: b a f n f a b 1/2 ( b a (f n f) 2 ) 1/2 a b 1/2 (2M 2 b a f n f ) 1/2.

3 The first one holds for any functions f n, f in (say) C pw [a, b]- it is the Cauchy- Schwarz inequality for integrals. It shows immediately that convergence in L 2 norm implies convergence in L 1 norm (on any bounded interval). The second one holds under the assumption that f and f n are bounded by M (just use (f n (x) f(x)) 2 2M f n (x) f(x) if f n M, f M). So it shows that if the sequence f n is uniformly bounded (i.e., all f n bounded by a constant M independent of n) and converges to f in the L 1 sense, then it also converges in the L 2 sense. (Note that, in this case f M also holds.) To get a better grasp of the distinction between these different notions of convergence, there is no substitute for the following exercise. Exercise 1. Consider the sequence of functions in [0, 1]: f n (x) = M n h 2 x 2 on [0, h n ], n f n (x) = M n h 2 (2h n x) 2 on [h n, 2h n ], n and f n (x) = 0 elsewhere in [0, 1]. We assume h n 0. M n is an arbitrary sequence of positive numbers. (i) Sketch the graphs of the f n. (ii) Show that f n 0 pointwise, regardless of how the M n are chosen; (iii) Show that f n 0 uniformly if M n 0, but not otherwise; (iv) Compute 1 0 f n and 1 0 f n 2 (use the symmetry!); (v) Show that if the sequence (M n ) is bounded, then f n 0 in L 2 norm (hence in L 1 norm). (vi) Show that, by suitably choosing M n, one can have examples with: (a) f n 0 in L 1 norm, but not in L 2 norm; (b) f n not converging to zero in L 1 norm. Part (vi)(a) shows that L 2 convergence to zero means a bit more than areas under the graph converge to zero. It is interesting that I had to use a quadratic function to get this example- it doesn t work with linear tents! Also, the examples of part (vi) cannot work if M n is bounded, as the following interesting proposition shows: Proposition. If f n f pointwise in [0, 1], and is uniformly bounded ( f n (x) M for all x, all n), then f n f in L 1 norm. (And hence also in L 2 norm- as noted above, under these hypotheses, L 1 convergence and L 2 convergence are equivalent). Proof. (Reader beware: The proof is recorded here for future reference, but following the details requires quite a bit more experience than I expect from the typical reader.) Try to understand the general idea, and feel free to ask me for more details if you are interested.) It is enough to consider the case f 0. Pick ɛ > 0. For each n, partition 3

4 [0, 1] into a good set and a bad set, [0, 1] = G n B n : B n = {x; f n > ɛ}, f n ɛ in G n. Denote by B n the measure ( generalized length ) of the set B n. If there is a subsequence n i such that B ni is bounded below (say, greater than some δ > 0), then these bad sets cannot all be disjoint, and one may take a further subsequence n i and find x [0, 1] so that x B ni for all i, that is, f ni (x) > ɛ for all i, which is not possible if f n (x) 0. Thus B n 0, and since f n M everywhere, we have: 1 0 f n = f n + B n f n M B n + G n f n M B n + ɛ 2ɛ, G n for n sufficiently large. Hence lim sup 1 0 f n 2ɛ, and since ɛ is arbitrary, it follows that 1 0 f n Fourier convergence theorems. Theorem 1. (Dirichlet 1824) Let f C pw. Then s N f pointwise on R, where f(x) := (f(x + ) + f(x ))/2 (average of the one-sided limits at x.) Theorem 2. Let f C per. Then s N f uniformly in R. Theorem 3. Let f C pw. Then s N f in L 2 norm. In practice Theorem 2 is the most powerful, but the proofs of all three results are interdependent in an interesting way. Be sure to review the definitions (above) of the spaces C per and C pw! Before we go on, do the following problem: Exercise 2. Let f be 2π-periodic, equal to zero everywhere except at 2nπ, where it equals 1. (i) what are the Fourier coefficients of f? (ii) To what function do the partial sums s N (x) converge? Is this pointwise convergence? Uniform? In L 2 norm? Are the answers in accordance with theorems 1-3 above? Decay of Fourier coefficients. The Fourier coefficients of a piecewise continuous function satisfy the obvious bound: a n, b n 1 π 4 π f(x) dx.

5 If f is continuous, with piecewise-continuous first-derivative (i.e., f C per ) integration by parts and periodicity yield: and hence the bound: a n [f] = 1 n b n[f ], b n [f] = 1 n a n[f ], a n, b n 1/π n f (x)dx. With f is more regular (has more continuous derivatives), we may iterate this procedure and obtain: f C k per a n, b n 1/π n k+1 f (k+1) (x) dx. To go further, we have to introduce a basic calculation in the L 2 theory. The symmetric bilinear form in C pw : f, g = f(x)g(x)dx has many of the properties of an inner product (generalized dot product ) in a finite-dimensional vector space. One crucial property fails: it is not positive-definite, but only positive semi-definite : f 2 := f, f, = 0 f = 0 except possibly on a set of measure 0. (For an inner product, v 2 = 0 v = 0). The usual problem that integration doesn t see sets of measure zero rears its ugly head. Still, the analogy is so useful that most people just ignore this, identify functions coinciding up to sets of measure zero without stopping to think about the consequences (e.g. it makes no sense to talk about the value of a function at a point), call it an inner product anyway and forge on ahead! In this spirit, consider the functions: e 0 = 1 2π, e n = 1 π cos nx, f n = 1 π sin nx. It is very easy to see that the e n (n 0) jointly with the f n (n 1) define an orthonormal set for the L 2 inner product. The Fourier coefficients have the expressions: a 0 = 2/π f, e 0, a n = (1/ π) f, e n, b n = (1/ π) f, f n, 5

6 while the partial sum s N (x) can be written as: s N (x) = f, e n e n + n=0 f, f n f n, which looks exactly like the formula for orthogonal projection onto a 2N +1- dimensional subspace of a vector space with an inner product, in terms of an orthonormal basis for the subspace! Encouraged by this, we compute the norm-squared of the remainder using the usual expansion for a quadratic form and find (using orthogonalitydo try this at home!): f s N 2 = f 2 or in terms of Fourier coefficients: f 2 π[a f, e n 2 n=0 f, f n 2, a 2 n + b 2 n] = f s N 2 0. This innocent-looking calculation has several important consequences: 1. ( Bessel s inequality ): the series n 0 a2 n and n 1 b2 n are convergent: a n 1(a 2 n + b 2 n) π 1 f ( Parseval s equality ): s N f in L 2 norm n 0 a 2 n + n 1 b 2 n = π 1 f ( Riemann-Lebesgue lemma ): a n and b n tend to zero as n tends to infinity, for any f C pw. This clearly follows from Bessel s inequality. We can apply this iteratively and get: f C k per n k+1 max{ a n, b n } 0, a stronger decay condition than that seen above. 6

7 Even more, this is all we need to give the proofs of theorems 2 and 3, assuming theorem 1. Proof of uniform convergence (Theorem 2). We assume theorem 1. Since, from theorem 1, we know s N (x) f(x) pointwise, we may write for each x R: s N (x) f(x) = a n cos nx + b n sin nx ( a n + b n ). n=n+1 N+1 To estimate the right-hand side, recall we are assuming f is piecewise C 1, so using the notation a n = a n [f ], b n = b n [f ], the right-hand side is bounded above by: n=n+1 1 n ( a n + b n ) 2( n=n+1 a n 2 + b n 2 ) 1/2 2( n=n n 2 )1/2 f 1/2, π where we used the Cauchy-Schwarz inequality for convergent series (and Bessel s inequality for f ). From the estimate for the tail part of a convergent series given by the integral test: n=n+1 1 n 2 N 1 x 2 dx = 1 N, we get not only uniform convergence, but also a rate of convergence: sup s N (x) f(x) ( 2 x R π f ) 1/2 1. N Proof of L 2 convergence (Theorem 3). We assume Theorem 2. The idea is to approximate f (which is only piecewise continuous) in L 2 norm by continuous functions, to which Theorem 2 can be applied. We need the following lemma: Lemma. If f is piecewise continuous (say, in [a, b]), with piecewise continuous derivative, then given ɛ > 0 we may find f ɛ continuous, with piecewise continuous derivative, so that f ɛ f < ɛ. In lieu of a proof, you are asked in Exercise 3 (below) to show this in a simple case (which contains the main idea for the general case) Given ɛ > 0, use the lemma to find f ɛ in C per with f f ɛ ɛ/4 7

8 (L 2 norm). Then use Theorem 2 (and the fact that uniform convergence implies convergence in L 2 ) to find N so that f ɛ is well-approximated in L 2 norm by the Nth. partial sum of its Fourier series: f ɛ s N [f ɛ ] ɛ/4. Finally, we need an estimate for the L 2 norm of s N [f ɛ ] s N [f]. Note that this is none other than the Nth. partial sum of the Fourier series of f ɛ f, to which the identity above (for the L 2 norm of the remainder) can be applied, yielding: f ɛ f s N [f ɛ f] f ɛ f ɛ/4. Using the triangle inequality, we have: Putting everything together, we find: s N [f ɛ f] 2 f ɛ f ɛ/2. f s N [f] f f ɛ + f ɛ s N [f ɛ ] + s N [f ɛ ] s N [f] ɛ 4 + ɛ 4 + ɛ 2 = ɛ, proving L 2 convergence (since ɛ is arbitrary). Exercise 3. Let f(x) be the piecewise continuous function in [, π]: f(x) = 1 for x (0, π], f(x) = 1 for x [, 0). Given h > 0, define f h in [, π] by: f h (x) = x h for x h, f h(x) = f(x) otherwise. (i) Sketch a graph of f and f h, verifying that f h is continuous; (ii) show (directly) that f f h 0 as h Proof of Dirichlet s theorem. The first step in the proof is a computation. Given f C pw, the Nth. partial sum of its Fourier series can be written as: s N (x) = 1 π ( 1 N 2 + cos nx cos ny + sin nx sin ny)f(y)dy. Using a standard trigonometric formula, we find an expression for s N (x) in convolution form : s N (x) = K N (x y)f(y)dy, 8

9 with K N (x) (the Dirichlet kernel) defined by: K N (x) = 1 N π (1 2 + cos(ny)). Amazingly, this sum can be computed explicitly! Using the De Moivre- Laplace formula to represent cos(nx) as a sum of complex exponentials, we find that K N (x) is (for x 0) the sum of 2N + 1 terms of a geometric sequence with ratio e ix : K N (x) = 1 2π n= N e nix = e(n+1)ix + e Nix e ix, x 0, 1 and the last expression can be written as a ratio of sines (multiplying numerator and denominator by e ix/2, yielding: K N (x) = 1 π sin[(n + 1/2)x], for x 0; K N (0) = 1 (2N + 1). 2 sin(x/2) π Note that it follows directly from the definition that: and by a change of variables: s N (x) = K N (x)dx = 1, f(x + y)k N (y)dy. Fixing x [, π], assume that f is continuous and differentiable at x (the proof is given only in this case). Then: s N (x) f(x) = where: g x (y) = [f(x+y) f(x)]k N (y)dy = 1 π f(x + y) f(x), y 0; g x (0) = f (x). 2 sin y/2 Note that g x is continuous and 2π-periodic. Now using: g x (y) sin[(n )y]dy, sin(n + 1/2)y = sin(ny) cos(y/2) + cos(ny) sin(ny/2), 9

10 we see that the integral may itself be identified as a sum of Fourier coefficients! (Namely, for c x : y g x (y) cos(y/2) and s x : y g x (y) sin(y/2):) s N (x) = b N [c x ] + a N [s x ], and therefore tends to zero, by the Riemann-Lebesgue lemma. Exercise 4. [Strauss, p.129] Consider the sequence of functions g n (x) in [0, 1] defined by: for n even: g n (x) = 1 in [3/4 1/n 2, 3/4 + 1/n 2 ), g n = 0 elsewhere; for n odd: g n (x) = 1 in [1/4 1/n 2, 1/4 + 1/n 2 ) g n = 0 elsewhere. Show that g n (x) 0 in L 2 norm, but not in the pointwise sense. Exercise 5. [Strauss, p. 130]. Consider the Fourier sine series of each of the functions below in [0, π]. Use the theorems above to discuss convergence of each of them in the uniform, pointwise and L 2 senses (in [0, π]). (a) f(x) = x 3 ; (b) f(x) = πx x 2 ; (c) f(x) = x Don t waste your time actually computing the Fourier coefficients; draw some informative diagrams instead. Exercise 6. [Strauss, p. 140] (Wirtinger s inequality) Show that if f is a C 1 function in [, π] and f(x)dx = 0, then: Hint: Use Parseval s equality! f 2 dx f 2 (x)dx. Later (in class) we will see a remarkable application of this fact. 10

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