sinc is not
The main results used in the proof will be that and the dominated convergence theorem.
Proof by contradiction:
Let and suppose it’s Lebesgue integrable in .
Consider the intervals and .
and the succession of functions , where is the characteristic function of the set .
Each is a continuous function of compact support and will thus be integrable in . Furthermore (pointwise)
in each , .
So
.
Suppose is integrable in . Then by the dominated convergence theorem .
But and we get the contradiction .
So cannot be integrable in .This implies that cannot be integrable in and since a function is integrable in a set iff its absolute value is