Yes, every continuous function on a closed interval [a, b] is Riemann integrable.
Functions with too many discontinuities, like the Dirichlet function, are not Riemann integrable.
JEE tests your understanding of definite integrals, properties, and application of basic theorems of the Riemann Integral.
The Riemann Integral sums "slices" vertically (function values), while the Lebesgue Integral sums horizontally (ranges of function values), making it more powerful for complex functions.
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Riemann Integral
The Riemann Integral is one of the most essential concepts in calculus and real analysis. It provides a rigorous way to define the area under a curve and serves as the foundation for more advanced topics in analysis and applied mathematics. Whether you’re preparing for competitive exams like JEE or diving into Real Analysis, mastering the Riemann integral is a must.
1.0Riemann Integral Definition
The Riemann Integral of a function f(x) over an interval [a, b] is the limit of the sum of areas of rectangles under the curve as the width of the rectangles approaches zero.
Formally, if f:[a,b]→⊠ is a bounded function, and we divide [a, b] into n subintervals:
a=x0<x1<x2<…<xn=b
Then for sample points xi∗∈[xi−1,xi], the Riemann sum is:
S=∑i=1nf(xi∗)Δxi
where Δxi=xi−xi−1. If the limit of these sums exists as max(Δxi)→0, it is called the Riemann Integral:
∫abf(x)dx
2.0Riemann Integral Formula
The general Riemann Integral formula is:
∫abf(x)dx=limn→∞∑i=1nf(xi∗)Δxi
Proof of Riemann Integral for f(x) = x over [0, 1]
Let’s divide [0, 1] into n equal parts, each of width Δx=n1, and use right endpoints:
In Real Analysis, the Riemann Integral is used to study the integration of bounded functions and understand properties like continuity, limits, and measure. It leads to more advanced theories like the Lebesgue Integral for handling more complex functions.
4.0Riemann Integral Properties
Linearity:
∫ab(af(x)+bg(x))dx=a∫abf(x)dx+b∫abg(x)dx
Additivity over intervals:
∫abf(x)dx=∫acf(x)dx+∫cbf(x)dx
Monotonicity:
If f(x)≤g(x) on [a,b], then ∫abf(x)dx≤∫abg(x)dx
Non-negativity:
If f(x)≥0 on [a, b], then ∫abf(x)dx≥0
5.0Upper and Lower Riemann Integral
Lower Sum (L(f,P)): Uses the minimum value of the function on each subinterval.
Upper Sum (U(f,P)): Uses the maximum value of the function on each subinterval.
If the supremum of all lower sums equals the infimum of all upper sums, the function is Riemann integrable.