Weierstrass M-test
If real functions fₙ on one set A satisfy |fₙ(x)|≤Mₙ for all x∈A, with Mₙ≥0 and ΣMₙ<∞, then Σfₙ converges uniformly and absolutely on A.
Key knowledge
If real functions fₙ on one set A satisfy |fₙ(x)|≤Mₙ for all x∈A, with Mₙ≥0 and ΣMₙ<∞, then Σfₙ converges uniformly and absolutely on A. Its uniform tail is bounded by the corresponding majorant tail.
Values and equations
sup_(x∈A)|Σ_(n>N) fₙ(x)|≤Σ_(n>N) Mₙ→0
- fₙ
- Real functions on the common set A
- Mₙ
- Nonnegative summable majorants
- N
- Tail cutoff
Conditions and scope
- The same summable bounds hold on the entire set, not separately chosen bounds depending on x.
- This sufficient test does not by itself justify term-by-term differentiation.
Sources · 1
References
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Evidence and review
Theorem 9.22 and uniform-Cauchy proof, printed page 177; absolute-convergence consequence, printed page 179. The same tail estimate applied to |fₙ| supplies uniform absolute convergence.
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