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Asymptotic behavior of functions, asymptotic series and related topics
10
votes
Alternating binomial Dirichlet series
Not an answer, but this may help with asymptotics:
According to Maple the o.g.f. for $H^s_n$ is
$$ \sum_{j=1}^\infty j^{-s} (-1)^{j-1} \sum_{n=j}^\infty {n \choose j} x^n
= {\frac {1}{-1+x}{\it polylog …
2
votes
Accepted
Behavior of a Solution of a Nonlinear ODE
There are really two separate cases:
$$\eqalign{a' + 1 &= +\sqrt{1 - k^2 a^2}\cr
a' + 1 &= -\sqrt{1 - k^2 a^2}\cr} $$
You could switch continuously from one to the other only on the curve …
8
votes
What is the series expression for (1+1/x)^x about x = \infty?
$\log f(z) = z \log(1+1/z) = \sum_{k=0}^\infty \frac{(-1)^k}{k+1} z^{-k}$ as $z \to +\infty$, so $f(z)$ is the exponential of this sum. See http://oeis.org/A055505 for the numerators and http://oeis. …
4
votes
Accepted
$\lim_{k\to\infty}{n\choose k}2^{1-{k\choose2}}$, where $n=\max\{n\in\mathbb{N}:{n\choose k}...
Let $N(k) = \max \{n \in \mathbb N: \; {n \choose k} 2^{1-{k\choose 2}} < 1 \}$.
Note that $$\frac{{n+1 \choose k}}{{n \choose k}} = \frac{n+1}{n+1-k}$$
so it suffices to prove that $N(k)/k \to \infty …
1
vote
Asymptotic Behaviour of Solutions to a Riccati-type ODE with Small Forcing Term
Maybe for some family of initial conditions (depending on the $O(1/r^4)$ term). But note that $f(r) = -1/r + c/r^2$ is a solution to $f'(r) = f(r)^2 - c^2/r^4$ with initial condition $f(1) = c-1$.
2
votes
Asymptotics for an exponential generating function from an ordinary
Using the gfun package in Maple, it appears that
$$ M(x) = -\frac{1}{4}-{
\frac {3\,x}{28}}-6 (x-7)\,\int_{0}^{x}\!{\frac {{{\rm e}^{t \left( 4\,\sqrt {3}+7
\right) }}}{ \left( t-7 \right) ^{2}}{\it …
5
votes
expression for infinite series with powers of factorial in denominator
If $\beta$ is an integer $\ge 3$, it can be written as a hypergeometric
$$ \mbox{$_0$F$_{\beta-1}$}(\ ;\, 1,\ldots,1;,\exp(c\beta))$$
where there are $\beta-1$ ones.
0
votes
Left tail of convex combinations of $\chi_1^2$
Let $X = \sum_{i=1}^n a_i Z_i^2$. If $m = \min(a_1,\ldots,a_n)$ and $M = \max(a_1,\ldots,a_n)$, we have $m A \le X \le M A$ where $A$ has $\chi^2$ distribution with $n$ degrees of freedom. Thus
$$\ …
3
votes
Infinite sum of asymptotic expansions
Of course it is possible, but much caution is required because we don't know
about uniformity in $k$ for the estimates corresponding to the $\sim$
statements. Counterexamples are very easy to const …
4
votes
Accepted
Functions defined as infinite products
According to Maple, the finite product $f(n) = \frac{\Gamma(n - (1 + \sqrt{1-4n})/2) \Gamma(n-(1-\sqrt{1-4n})/2) \Gamma(1-\sqrt{-n}) \Gamma(1+\sqrt{-n})}
{\Gamma(n-\sqrt{-n}) \Gamma(n+\sqrt{-n}) \Gamm …
3
votes
Solution of a 2D Recurrence sequence
If $P_k(t) = \sum_{m=0}^k a_{m,k-n} t^m$ is the generating function of an ascending antidiagonal, we have
$$P_k(t) = \frac{t^k-t}{t-1} + \frac{1+t}{2} P_{k-1}(t), \ P_0(t) = 0 $$
and this can be solv …
4
votes
Simple bound on $\log(x)/x$
If you want the smallest, try
$$x = -LambertW(-\epsilon)/\epsilon = 1+\epsilon+{\frac{3}{2}}{\epsilon}^{2}+{\frac{8}{3}}{\epsilon}^{3}+{\frac{125}{24}}{\epsilon}^{4}
+O \left( {\epsilon}^{5} \right)
…