Math Problem Statement

if f is continuous and strictly positive on [0, 1], compute lim x→0+ x ∫ 1 x f (t) t2 dx

Solution

We are given a function ff that is continuous and strictly positive on the interval [0,1][0, 1], and asked to compute the following limit:

limx0+xx1f(t)t2dt\lim_{x \to 0^+} x \int_{x}^{1} \frac{f(t)}{t^2} \, dt

Step-by-Step Solution:

  1. Rewrite the expression:
    We need to evaluate the limit of the product of xx and the integral:

    xx1f(t)t2dtx \int_{x}^{1} \frac{f(t)}{t^2} \, dt

  2. Look for the behavior as x0+x \to 0^+:
    Since f(t)f(t) is continuous and strictly positive on [0,1][0, 1], it implies that as x0+x \to 0^+, f(t)f(t) will behave nicely and will not approach zero or infinity in an unbounded way. Let's break down the behavior of the integrand f(t)t2\frac{f(t)}{t^2} near t=0t = 0.

  3. Estimate the integral:
    Since f(t)f(t) is continuous on [0,1][0, 1], f(t)f(t) is bounded, meaning there exists a constant M>0M > 0 such that for all t[0,1]t \in [0, 1], f(t)Mf(t) \leq M. Therefore, we can estimate the integral:

    x1f(t)t2dtx1Mt2dt\int_{x}^{1} \frac{f(t)}{t^2} \, dt \leq \int_{x}^{1} \frac{M}{t^2} \, dt

  4. Evaluate the integral:
    The integral 1t2dt=1t\int \frac{1}{t^2} \, dt = -\frac{1}{t}, so:

    x1Mt2dt=M(1t)x1=M(11+1x)=M(1x1)\int_{x}^{1} \frac{M}{t^2} \, dt = M \left( -\frac{1}{t} \right) \Big|_x^1 = M \left( -\frac{1}{1} + \frac{1}{x} \right) = M \left( \frac{1}{x} - 1 \right)

  5. Combine with the factor xx:
    Now, multiply this by xx:

    xx1f(t)t2dtxM(1x1)=M(1x)x \int_{x}^{1} \frac{f(t)}{t^2} \, dt \leq x M \left( \frac{1}{x} - 1 \right) = M \left( 1 - x \right)

  6. Take the limit as x0+x \to 0^+:
    As x0+x \to 0^+, M(1x)MM(1 - x) \to M. Therefore, the integral behaves like:

    limx0+xx1f(t)t2dt=M\lim_{x \to 0^+} x \int_{x}^{1} \frac{f(t)}{t^2} \, dt = M

  7. Conclusion:
    Since f(t)f(t) is continuous and strictly positive, we conclude that the limit is MM, the maximum value of f(t)f(t) as tt approaches 0.

Thus, the final result is:

M\boxed{M}

where MM is a constant such that f(t)f(t) is bounded by MM over the interval [0,1][0, 1].


Do you need further details or clarifications on any steps?

Here are 5 related questions that can expand your understanding:

  1. How can the continuity and positivity of f(t)f(t) affect the behavior of the integral in different scenarios?
  2. What is the significance of the bound MM in this problem?
  3. Can you derive the asymptotic behavior of xx1f(t)t2dtx \int_{x}^{1} \frac{f(t)}{t^2} \, dt for different types of functions f(t)f(t)?
  4. How does the choice of the limits in the integral influence the result when taking the limit as x0+x \to 0^+?
  5. What happens to the integral if f(t)f(t) has singularities or is not bounded near 0?

Tip: When faced with limits involving integrals, consider bounding the integrand to estimate its behavior, especially if the function involved is continuous or has known properties over the interval.

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Math Problem Analysis

Mathematical Concepts

Limits
Integrals
Continuity
Positivity of functions

Formulas

lim_{x → 0+} x ∫_{x}^{1} f(t) / t^2 dt

Theorems

Theorem of continuity and boundedness of functions
Fundamental Theorem of Calculus

Suitable Grade Level

Undergraduate (Calculus I)