Show that the function f:[0,4] by fx=0 where x is rational and fx=2 where x is irrational is not remann integrable but is lesberg integrable
Solution:
Given function is same as, f is 0 at every rational number and 2 at every irrational number.
If P={I1,I2,…,In} is a partition of [0,4], then
since every interval of non-zero length contains both rational and irrational numbers. It follows that
for every partition P of [0,1], so U(f)=1 and L(f)=0 are not equal.
Hence, this function is not Riemann integrable.
Further, the given function is clearly Lebesgue-integrable as being equal to 0 almost everywhere.
Let . Then f(x)=2. Pick a sequence of irrational numbers . Then , so f is not continuous in x.
Let . Then f(x)=0. Pick a sequence of rational numbers . Then , so f is not continuous in x.
The above argument shows that f is discontinuous in all the points of [a, b], which is a set of Lebesgue measure , therefore f is lebesgue integrable on [a, b].
Hence, proved.
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