Asymptotics of roots of equations and implicit functions

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Show that the equation

x(1+log⁡(1εx))=1,x>0,ε>0, x\left(1+\log \left(\frac{1}{\varepsilon \sqrt{x}}\right)\right) = 1, \qquad x > 0, \quad \varepsilon > 0,

has, for each sufficiently small ε>0\varepsilon > 0, exactly two solutions. Let x(ε)x(\varepsilon ) be the smaller one. Show that

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(1)

x(ε)→0x(\varepsilon ) \rightarrow 0 as ε→0+\varepsilon \rightarrow 0+;

(2)

yet for any s>0s > 0, ε−sx(ε)→∞\varepsilon^{-s} x(\varepsilon ) \rightarrow \infty as ε→0+\varepsilon \rightarrow 0+.

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(1)

Prove that the equation tg⁡x=a/x,a>0\operatorname {tg} x = a / x, a > 0, has exactly one solution on each interval (−π/2+πn;π/2+πn),n∈N(-\pi / 2+\pi n ; \pi / 2+\pi n), n \in \mathbb {N}.

(2)

Let xnx_{n} be the solution of the equation tg⁡x=a/x,a>0\operatorname {tg} x = a / x, a > 0, lying in the interval (−π/2+πn;π/2+πn),n∈N(-\pi / 2+\pi n ; \pi / 2+\pi n), n \in \mathbb {N}. Prove that

0<xn−πn<2aπn+π2n2+4a,n∈N 0 < x_{n}-\pi n < \frac{2 a}{\pi n+\sqrt{\pi ^{2} n^{2}+4 a}}, \quad n \in \mathbb {N}
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(1)

Prove that the equation tg⁡x=ax,a>0\operatorname {tg} x = a x, \quad a > 0, has exactly one solution xnx_{n} on each interval (−π/2+πn;π/2+πn),n∈N(-\pi / 2+\pi n ; \pi / 2+\pi n), n \in \mathbb {N}.

(2)

Let xnx_{n} be the solution of the equation tg⁡x=ax,a>0\operatorname {tg} x = a x, a > 0, lying in the interval (−π/2+πn;π/2+πn),n∈N(-\pi / 2+\pi n ; \pi / 2+\pi n), n \in \mathbb {N}. Prove that for all sufficiently large n∈Nn \in \mathbb {N}

0<π2+πn−xn<2/aπ/2+πn+(π/2+πn)2−4/a. 0 < \frac{\pi }{2}+\pi n-x_{n} < \frac{2 / a}{\pi / 2+\pi n+\sqrt{(\pi / 2+\pi n)^{2}}-4 / a}.
(3)

Find lim⁡n→∞(xn+1−xn)\lim_{n \rightarrow \infty }\left(x_{n+1}-x_{n}\right), where the sequence {xn}\left\{ x_{n}\right\} is defined in 1) and 2).

Let {xn}\left\{ x_{n}\right\} be the increasing sequence of positive roots of the equation xsin⁡x=1x \sin x = 1. Prove that as n→∞n \rightarrow \infty:

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(1)

xn=πn+o(1)x_{n} = \pi n+o(1);

(2)

xn=πn+(−1)n/(πn)+o(1/n)x_{n} = \pi n+(-1)^{n} /(\pi n)+o(1 / n);

(3)

xn=πn+(−1)n/(πn)−(1/(π3n3))(1−(−1)n/6)+o(1/n3)x_{n} = \pi n+(-1)^{n} /(\pi n)-\left(1 /\left(\pi^{3} n^{3}\right)\right)\left(1-(-1)^{n} / 6\right)+o\left(1 / n^{3}\right).

Let {xn}\left\{ x_{n}\right\} be the increasing sequence of positive roots of the equation tg⁡x=1/(1+x2)\operatorname {tg} x = 1 /\left(1+x^{2}\right). Prove that as n→∞n \rightarrow \infty

xn=πn+1/(πn)2+o(1/n2). x_{n} = \pi n+1 /(\pi n)^{2}+o\left(1 / n^{2}\right).
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Let {xn}\left\{ x_{n}\right\} be the increasing sequence of positive roots of the equation tg⁡x=x\operatorname {tg} x = x. Prove that as n→∞n \rightarrow \infty:

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(1)

xn=π2+πn+o(1)x_{n} = \frac{\pi }{2}+\pi n+o(1);

(2)

xn=π2+πn−1π/2+πn+o(1n)x_{n} = \frac{\pi }{2}+\pi n-\frac{1}{\pi / 2+\pi n}+o\left(\frac{1}{n}\right);

(3)

xn=π2+πn−1π/2+πn−23(π/2+πn)3+o(1n3)x_{n} = \frac{\pi }{2}+\pi n-\frac{1}{\pi / 2+\pi n}-\frac{2}{3(\pi / 2+\pi n)^{3}}+o\left(\frac{1}{n^{3}}\right).

Let y(x)=∫x+∞e−t2/2dt(x∈R),x(y)y(x)=\int_{x}^{+\infty } e^{-t^{2} / 2} d t(x \in \mathbb {R}), x(y) be the function inverse to y(x)y(x). Prove that x(y)∼y→+02ln⁡1yx(y) \underset {y \rightarrow +0}{\sim } \sqrt{2 \ln \frac{1}{y}}.

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Verify that the domain of definition of the implicit functions considered in problem 4.2 contains the half-line [x0,+∞)\left[x_{0},+\infty \right), and prove that as x→+∞x \rightarrow +\infty

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(a)

y(x)=ln⁡x−ln⁡ln⁡x+c1a1(x)+c2a2(x)+O(a2(x)ln⁡ln⁡x)y(x)=\ln x-\ln \ln x+c_{1} a_{1}(x)+c_{2} a_{2}(x)+O\left(\frac{a_{2}(x)}{\ln \ln x}\right);

(b)

y(x)=x(c0+c1a1(x)+c2a2(x)+O(a2(x)ln⁡x))y(x)=\sqrt{x}\left(c_{0}+c_{1} a_{1}(x)+c_{2} a_{2}(x)+O\left(\frac{a_{2}(x)}{\ln x}\right)\right);

(c)

y(x)=ln⁡xx+1x(c1a1(x)+c2a2(x)+O(a2(x)ln⁡x))y(x)=\frac{\ln x}{x}+\frac{1}{x}\left(c_{1} a_{1}(x)+c_{2} a_{2}(x)+O\left(\frac{a_{2}(x)}{\ln x}\right)\right);

(d)

y(x)=xln⁡x(c0+c1a1(x)+c2a2(x)+O(a2(x)ln⁡ln⁡x))y(x)=\frac{x}{\ln x}\left(c_{0}+c_{1} a_{1}(x)+c_{2} a_{2}(x)+O\left(\frac{a_{2}(x)}{\ln \ln x}\right)\right), where c0,c1,c2c_{0}, c_{1}, c_{2} are constants, and aka_{k} are functions of the form (ln⁡x)mxn\frac{(\ln x)^{m}}{x^{n}} or (ln⁡ln⁡x)mln⁡nx\frac{(\ln \ln x)^{m}}{\ln^{n} x} for suitable m,n∈Nm, n \in \mathbb {N};

(e)

y(x)=−x2+1−ex−x3+O(xe2(x−x3))y(x)=-x^{2}+1-e^{x-x^{3}}+O\left(x e^{2\left(x-x^{3}\right)}\right);

(f)

y(x)=−x2+π4−1x+O(1x2)y(x)=-\frac{x}{2}+\frac{\pi }{4}-\frac{1}{x}+O\left(\frac{1}{x^{2}}\right).

Prove that the equation yx+y=1y^{x}+y=1 defines, for all x>0x>0, a unique implicit function yy and that

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(a)

y∈C∞((0,∞)),0<y<1,y′>0y \in C^{\infty }((0, \infty )), 0<y<1, y^{\prime }>0;

(b)

y(x)=x∣ln⁡x∣+xln⁡∣ln⁡x∣+o(x)y(x)=x\left|\ln x\right|+x \ln \left|\ln x\right|+o(x) as x→+0x \rightarrow +0;

(c)

y(x)=1−ln⁡xx+ln⁡ln⁡xx+o(1x)y(x)=1-\frac{\ln x}{x}+\frac{\ln \ln x}{x}+o\left(\frac{1}{x}\right) as x→+∞x \rightarrow +\infty.

How many continuous solutions yy does the equation x5+x2y=y3x^{5}+x^{2} y=y^{3} have that tend to zero as x→0x \rightarrow 0? What is their asymptotic behavior as x→0x \rightarrow 0?

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The problem of "the globe in a string bag." The Earth's globe was girdled with an inextensible thread. Then the thread was lengthened by 1 cm and pulled taut by tugging at the knot. Compute the height to which it rose, with a relative error of no more than one ten-thousandth of a percent (take the radius of the globe to be 6400 km6400\ \text{km}).

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Let xnx_{n} be the root of the equation x=tg⁡xx=\operatorname {tg} x lying in the interval (πn,πn+π\pi n, \pi n+\pi), n∈Nn \in \mathbb {N}. Prove that

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(a)

xn=π(n+12)−1πn+O(1n2)x_{n}=\pi \left(n+\frac{1}{2}\right)-\frac{1}{\pi n}+O\left(\frac{1}{n^{2}}\right);

(b)

−1π(n+12)−1(πn)3<xn−π(n+12)<−1π(n+12)-\frac{1}{\pi \left(n+\frac{1}{2}\right)}-\frac{1}{(\pi n)^{3}}<x_{n}-\pi \left(n+\frac{1}{2}\right)<-\frac{1}{\pi \left(n+\frac{1}{2}\right)}.

Suppose that α\alpha is positive and that xnx_{n} is the (only) root of the equation

12x+1x−1+1x−2+⋯+1x−n=α \frac{1}{2x} + \frac{1}{x-1} + \frac{1}{x-2} + \cdots + \frac{1}{x-n} = \alpha

in the interval (n,∞)(n, \infty ). Prove that

lim⁡n→∞(xn−n+121−e−α)=0.(12.) \lim _{n \to \infty } \left(x_{n} - \frac{n + \frac{1}{2}}{1 - e^{-\alpha }}\right) = 0 . \tag {12.}
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Assume that α\alpha is positive and that xn′x_{n}' is the (only) root in the interval (n,∞)(n, \infty ) of the equation

1x+2xx2−12+2xx2−22+⋯+2xx2−n2=α. \frac{1}{x} + \frac{2x}{x^{2} - 1^{2}} + \frac{2x}{x^{2} - 2^{2}} + \cdots + \frac{2x}{x^{2} - n^{2}} = \alpha .

Verify that

lim⁡n→∞(xn′−(n+12)1+e−α1−e−α)=0.(12.) \lim _{n \to \infty } \left(x_{n}' - \left(n + \frac{1}{2}\right) \frac{1 + e^{-\alpha }}{1 - e^{-\alpha }}\right) = 0 . \tag {12.}
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We denote by λ\lambda a positive proper fraction and by xnx_{n} the only positive root of the transcendental equation

1+x1!+x22!+⋯+xnn!=λex 1 + \frac{x}{1!} + \frac{x^{2}}{2!} + \cdots + \frac{x^{n}}{n!} = \lambda e^{x}

As n→∞n \to \infty the root is given by

xn=n+αn+β+o(1), x_{n} = n + \alpha \sqrt{n} + \beta + o(1),

where α\alpha and β\beta satisfy the equations:

12π∫α∞e−t22 dt=λ,β=α2+23. \frac{1}{\sqrt{2\pi }} \int _{\alpha }^{\infty } e^{-\frac{t^{2}}{2}} \, dt = \lambda , \qquad \beta = \frac{\alpha ^{2}+2}{3} .
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Remark.
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Problem 42 of Part V.

Suppose that nn is odd and let −xn-x_{n} denote the only real root of the equation

1+x1!+x22!+⋯+xnn!=0 1 + \frac{x}{1!} + \frac{x^{2}}{2!} + \cdots + \frac{x^{n}}{n!} = 0

As n→∞n \to \infty, xnx_{n} is asymptotically given by

xn=ξn+αlog⁡n+β+o(1), x_{n} = \xi n + \alpha \log n + \beta + o(1),

where ξ\xi is the only real root of the equation e1+ξξ=1e^{1+\xi }\xi = 1 and α\alpha and β\beta are given by

α=12ξ1+ξ,β=ξ1+ξlog⁡(2π1+ξξ). \alpha = \frac{1}{2}\frac{\xi }{1+\xi }, \qquad \beta = \frac{\xi }{1+\xi } \log \left(\sqrt{2\pi }\frac{1+\xi }{\xi }\right) .
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Remark.
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Problem 74 of Part V.

References

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