Home›Calculus 2›Radius of convergence›Problem 7.119

Radius of convergence

Problem 7.119 · easy

Find the radius of convergence of \( \displaystyle \sum_{n=1}^{\infty} \frac{\left(x - 2\right)^{n}}{n!} \).

Use the ratio test on the absolute values of the terms.
\[ \lim_{n \to \infty}\left(\frac{\left(x - 2\right)^{- n} \left(x - 2\right)^{n + 1} n!}{\left(x - 2\right) \left(n + 1\right)!}\right) = 0 \]
|a_(n+1)/a_n| → 0·|x − 2|.✓ Proved
The series converges when 0|x − 2| < 1, which is every x: R = ∞.

Answer \( R = \infty \)

Lines: 1 proved, 2 not checked. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt. How we verify

The full receipt

Line	Status	Checked by	Detail
1	Not checked	—	a sentence; read, not computed
2	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
3	Not checked	—	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: fail (error) — The solution incorrectly applies the convergence condition for the Ratio Test. The test states that a series converges if the limit L < 1. Since the limit is 0 for all x, the condition 0 < 1 is always satisfied, meaning the series converges for all x. The solution erroneously sets up the inequality 0|x-2| < 1, implying a dependency on x that does not exist in the limit result, and confusingly mixes the form of the limit with the convergence criterion.

Every verdict on record (6)

qwen3.6:27b-mlx: fail (error) 2026-09-28 — The solution incorrectly applies the convergence condition for the Ratio Test. The test states that a series converges if the limit L < 1. Since the limit is 0 for all x, the condition 0 < 1 is always satisfied, meaning the series converges for all x. The solution erroneously sets up the inequality 0|x-2| < 1, implying a dependency on x that does not exist in the limit result, and confusingly mixes the form of the limit with the convergence criterion.
gpt-oss:20b: pass 2026-09-28
qwen3.6:27b-mlx: fail (error) 2026-09-27 — The solution incorrectly states the convergence condition as 0|x - 2| < 1. The Ratio Test requires the limit L to be strictly less than 1 for convergence; since L = 0 for all x, the condition is 0 < 1, which is always true, not dependent on |x - 2|.
gpt-oss:20b: pass 2026-09-27
qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly applies the ratio test and derives the correct radius of convergence.
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 2026-09-28 with SymPy 1.14.0.