414/514 Simple examples of Baire class one functions

Recall that a real-valued function f defined on an interval I is (in) Baire class one (\mathcal B_1) iff it is the pointwise limit of continuous functions.

Examples are continuous functions, of course, but functions in \mathcal B_1 do not need to be continuous. An easy example is the function f:[0,1]\to\mathbb R given by f(x)=0 if x\ne 1 and f(x)=1 if x=1. This is the pointwise limit of the functions f_n(x)=x^n. By the way, an easy modification of this example shows that any function that is zero except at finitely many points is in \mathcal B_1.

Step functions are another source of examples. Suppose that a=x_0<x_1<\dots<x_{n-1}<x_n=b and that s:[a,b]\to\mathbb R is constant on each (x_i,x_{i+1}). Then s is the pointwise limit of the functions s_k, defined as follows: Fix a decreasing sequence \epsilon_k converging to 0, with \epsilon_k\le 1/k and 2\epsilon_k<x_{i+1}-x_i for all i. Now define \hat s_k as the restriction of f to

\displaystyle \{x_0,x_1,\dots,x_n\}\cup\bigcup_{i=0}^{n-1}[x_i+\epsilon_k,x_{i+1}-\epsilon_k],

and let s_k:[a,b]\to\mathbb R extend \hat s_k by joining consecutive endpoints of the components of its domain with straight segments.

An important source of additional examples is the class of derivatives. Suppose f:\mathbb R\to\mathbb R and f(x)=g'(x) for all x. This is the pointwise limit of the functions f_n(x) given by

f_n(x)=\displaystyle\frac{g\left(x+\frac1n\right)-g(x)}{\frac1n}.

This simple construction does not quite work if f is defined on a bounded interval (as x+1/n may fall outside the interval for some values of x). We can modify this easily by using straight segments as in the case of step functions: Say f:[a,b]\to\mathbb R. For n large enough so 1/n<b-a, define f_n(x) as above for x\in[a,b-1/n], and now set f_n(b)=f(b) and extend f_n linearly in the interval {}[b-1/n,b].

Additional examples can be obtained by observing, first, that \mathcal B_1 is a real vector space, and second, that it is closed under uniform limits (the latter is not quite obvious). This gives us, for instance, that all monotone functions are in \mathcal B_1, since monotone functions are the uniform limit of step functions on bounded intervals: Given an increasing f:[a,b]\to\mathbb R, let f_n(x)=\lfloor nf(x)\rfloor/n. It follows that all functions of bounded variation are in \mathcal B_1, since any such function is the difference of two increasing functions.

Another interesting source of examples is characteristic functions. Given X\subseteq\mathbb R, the function \chi_X is in \mathcal B_1 iff X is both an \mathbf F_\sigma and a \mathbf G_\delta set.

On the other hand, \chi_{\mathbb Q} is not in \mathcal B_1, since it is discontinuous everywhere while Baire class one functions are continuous on a comeager set.

6 Responses to 414/514 Simple examples of Baire class one functions

  1. Thanks to Stuart Nygard for suggesting the much easier argument for derivatives being \mathcal B_1 than the messier approach I suggested in lecture.

    (Proofs of the closure of \mathcal B_1 under uniform limits, of the continuity fact, and of the claim about characteristic functions, will be provided in lecture.)

  2. Monica says:

    Sorry I am a bit confused .. Is this saying that we are setting f_n(b) = f(b) = g'(b)? So by taking it as the derivative, it approaches b but never passes it, or touches it?

    • Hi Monica.

      By construction, each f_n is continuous.

      For any \epsilon>0, if n is large enough then 1/n<\epsilon. If x<b, letting \epsilon=b-x, we see that for all n sufficiently large, \displaystyle f_n(x)=\frac{g\left(x+\frac1n\right)-g(x)}{\frac1n}, and this expression converges to g'(x)=f(x).

      So the only issue with this definition is whether we also have f_n(b)\to f(b), but we arrange that this happens trivially, by setting f_n(x)=f(b) for all n.

      Putting all this together, we see that f_n\to f pointwise.

  3. Monica says:

    oh okay thanks Andres!

  4. […] Previously, we listed some examples of Baire class one functions. Here we do the same for functions in the next class of Baire. Recall that if is an interval, the function is (in) Baire class two () iff it is the pointwise limit of a sequence of Baire one functions. […]

  5. […] derivatives are Darboux continuous (that is, they satisfy the intermediate value property), and are Baire one functions (that is, they are the pointwise limit of a sequence of continuous functions). But this […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: