Math Problem Statement

Prove the inequality: For all natural numbers n, 1 + 1/2² + 1/3² + … + 1/n² ≤ 2 - 1/n.

Solution

The problem provided asks to prove the inequality for all natural numbers nn:

1+122+132++1n221n.1 + \frac{1}{2^2} + \frac{1}{3^2} + \dots + \frac{1}{n^2} \leq 2 - \frac{1}{n}.

Steps to prove the inequality:

  1. Express the sum: The sum on the left-hand side is a partial sum of the series: Sn=1+122+132++1n2.S_n = 1 + \frac{1}{2^2} + \frac{1}{3^2} + \dots + \frac{1}{n^2}. This is the sum of the first nn terms of the series: k=1n1k2.\sum_{k=1}^{n} \frac{1}{k^2}.

  2. Upper Bound of the Series: It is known that the sum of the infinite series: k=11k2=ζ(2)=π26.\sum_{k=1}^{\infty} \frac{1}{k^2} = \zeta(2) = \frac{\pi^2}{6}. Numerically, ζ(2)1.64493\zeta(2) \approx 1.64493, which is less than 2.

    So, for all nn, we can already see that the infinite sum is bounded by a constant less than 2. However, we are interested in bounding the partial sum SnS_n.

  3. Analysis of the Bound: The given inequality suggests that for each natural number nn, the partial sum SnS_n is bounded by: Sn21n.S_n \leq 2 - \frac{1}{n}. Since the series converges to π26\frac{\pi^2}{6} as nn \to \infty, and 21n22 - \frac{1}{n} \to 2, this is reasonable because the partial sum should approach a limit less than 2, but it must also be corrected for the decreasing size of the terms.

  4. Induction Hypothesis: To formally prove the inequality, we can use mathematical induction or comparison of the terms of the series with decreasing fractions, showing that the difference between the sum and 2 becomes smaller as nn increases.


Would you like a more detailed walkthrough of each step, including an induction proof or comparison analysis?

Here are 5 questions that might deepen your understanding:

  1. What is the significance of the convergence of the infinite sum ζ(2)\zeta(2) in this inequality?
  2. How does the term 1n\frac{1}{n} act as a correction factor in the inequality?
  3. Could a more precise upper bound for the partial sum SnS_n be computed?
  4. How does the difference between SnS_n and π26\frac{\pi^2}{6} behave for large nn?
  5. What other techniques could be used to prove inequalities involving sums of series?

Tip: For series and inequalities, combining known results from convergence can simplify proofs significantly.

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Math Problem Analysis

Mathematical Concepts

Series and Summations
Inequalities
Limits and Convergence

Formulas

Harmonic series: S_n = Σ (1/k²) for k=1 to n
Euler's zeta function for k=2, ζ(2) = π²/6

Theorems

Convergence of the series Σ 1/k² to ζ(2)
Induction proof for inequalities

Suitable Grade Level

University level or advanced high school mathematics