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Tony's Intertwingled Memex • Reference Manual

Improper Integrals

Formulas & Definitions
Core Definition

An integral where either the interval of integration is infinite (Type I), or the integrand has an infinite discontinuity (vertical asymptote) on or within the interval [a,b][a, b] (Type II). Formally defined as a limit of proper definite integrals:

∫a∞f(x) dx=lim⁡t→∞∫atf(x) dx\int_{a}^{\infty} f(x) \, dx = \lim_{t \to \infty} \int_{a}^{t} f(x) \, dx

Concept Explanation

If the limit defining an improper integral exists and is finite, the integral converges. If the limit fails to exist or approaches infinity, it diverges. This tests whether infinite boundaries enclose a finite area.

Visual / Geometric Intuition

Interactive Desmos Graph

Engineering Applications

Improper integration is crucial in aerospace engineering to determine a rocket's planetary escape velocity (integrating gravity out to infinity), modeling radioactive decay, and calculating electrical power distributions in signal processing using Laplace transforms.

Example Problem

Evaluate the improper integral, or show that it diverges:

∫1∞1x2 dx\int_{1}^{\infty} \frac{1}{x^2} \, dx

Solution

Step 1: Rewrite the integral as a limit

∫1∞1x2 dx=lim⁡t→∞∫1tx−2 dx\int_{1}^{\infty} \frac{1}{x^2} \, dx = \lim_{t \to \infty} \int_{1}^{t} x^{-2} \, dx

Step 2: Integrate

=lim⁡t→∞[−x−1]1t=lim⁡t→∞(−1t−(−1))= \lim_{t \to \infty} \Big[ -x^{-1} \Big]_1^t = \lim_{t \to \infty} \left( -\frac{1}{t} - (-1) \right)

Step 3: Evaluate the limit

=lim⁡t→∞(1−1t)=1−0=1= \lim_{t \to \infty} \left( 1 - \frac{1}{t} \right) = 1 - 0 = 1

Since the limit is finite, the integral converges to 11.

Connections & References