Home›Calculus 2›Radius of convergence›Problem 7.141

Radius of convergence

Problem 7.141 · easy

Find the radius of convergence of \( \displaystyle \sum_{n=1}^{\infty} 4^{- n} x^{n} \).

Use the ratio test on the absolute values of the terms.
Reviewed
\[ \lim_{n \to \infty}\left(\frac{4^{n} 4^{- n - 1} x^{- n} x^{n + 1}}{x}\right) = \frac{1}{4} \]
|a_(n+1)/a_n| → 1/4·|x − 0|.✓ Proved
The series converges when 1/4|x − 0| < 1, that is |x − 0| < 4.
Reviewed

Answer \( R = 4 \)

✓ Nihil obstat Lines: 1 proved, 2 reviewed. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.

The full receipt

Line	Status	Checked by	Detail
1	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
2	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
3	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	terms evaluated just inside and just outside the radius shrink and grow

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly applies the ratio test to determine the radius of convergence. The limit calculation and the resulting inequality are accurate.

Every verdict on record (4)

qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly applies the ratio test to determine the radius of convergence. The limit calculation and the resulting inequality are accurate.
gpt-oss:20b: pass 2026-09-26
qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly applies the ratio test to determine the radius of convergence. The limit calculation and the resulting inequality are correct.
gpt-oss:20b: pass 2026-09-26

Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence, not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by generator:structured/radius_of_convergence, checked 2026-09-26 with SymPy 1.14.0.