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Convergence of series, sequences and functions and different modes of convergence.

18 votes
Accepted

Asymptotics of a Bernoulli-number-like function

[Revised and expanded to give the answer for all $k>1$ and incorporate further terms of an asymptotic expansion as $n \rightarrow \infty$] Fix $k>1$, and write $a_1=f(1,k)=1$ and $$ a_n = f(n,k) = \f …
Noam D. Elkies's user avatar
12 votes

Probability of winning game whereby $T+1$ heads in a row of a coin flip is required to win w...

The probability of not winning is $$ \prod_{T=1}^\infty \left(1 - \frac1{2^T} \right) = \frac12 \frac34 \frac78 \frac{15}{16} \cdots = 0.28878809508660 \ldots ; $$ that's a well-known constant (equal …
Noam D. Elkies's user avatar
8 votes

Asymptotic behavior of a certain trigonometric partial sum

The desired inequality should be true iff $$ c < c_0 := (r - \sqrt{r^2-1})^2 \quad\ \text{where} \quad\ r = \frac{|a|}{2b} $$ (NB the hypotheses $b>0$ and $a < -2b$ imply $r>1$, so $0 < c_0 < 1$). Num …
Noam D. Elkies's user avatar
6 votes

On the continuity of $\sum_{n=1}^{\infty} \sin(nx) / n^\alpha$

For small $x>0$ we can write $$ f(x) = x^{\alpha-1} \cdot x \sum_{n=1}^\infty \frac{\sin nx}{(nx)^\alpha}, $$ which is $x^{\alpha-1}$ times a Riemann sum for $$ I_\alpha := \int_0^\infty \sin u \frac{ …
Noam D. Elkies's user avatar