Power series from the geometric series
Problem 7.487 · medium
Find a power series for \( \displaystyle f(x) = \frac{1}{3 - x} \) centered at 0, and its interval of convergence.
- \[ 1 \frac{1}{3 - x} = \frac{1}{3 - x} \]Write f in the form (something)/(1 − u) with u = x/3.✓ Proved
- 1/(1 − u) = Σ uⁿ for |u| < 1; substitute and multiply through.Reviewed
- \[ \lim_{x \to 0^+}\left(\frac{- \frac{x^{3}}{81} - \frac{x^{2}}{27} - \frac{x}{9} - \frac{1}{3} + \frac{1}{3 - x}}{x^{3}}\right) = 0 \]The first terms x**3/81 + x**2/27 + x/9 + 1/3 agree with f to order x^3.✓ Proved
- |u| < 1 means |x| < 3; the series diverges at both endpoints (the terms do not shrink).Reviewed
Answer \( \sum_{n=0}^{\infty} 3^{- n - 1} x^{n},\quad |x| < 3 \)
✓ Nihil obstat Lines: 2 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 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 4 | 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 | the series summed numerically at half the radius matches f |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly derives the power series and interval of convergence. The reasoning for endpoint divergence is sound, as the terms approach a non-zero constant (±1/3) rather than zero.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-10qwen3.6:27b-mlx: pass 2026-10-10 — The solution correctly derives the power series and interval of convergence. The reasoning for endpoint divergence is sound, as the terms approach a non-zero constant (±1/3) rather than zero.gpt-oss:20b: pass 2026-10-10qwen3.6:27b-mlx: pass 2026-10-10 — The solution correctly derives the power series and interval of convergence. The reasoning for divergence at endpoints is sound (terms do not approach zero).
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/power_series_geometric, checked 2026-10-10 with SymPy 1.14.0.