Is $int_0^{+infty}frac{sin ln x}{sqrt x},dx$ convergent or absolutely convergent?
$begingroup$
Is
$$int_0^inftyfrac{sinln x}{sqrt x},dx$$
convergent? Absolutely convergent?
I found, that in fact it converges, but the only way I found is to say that $$int_0^{+infty}frac{sin ln x}{sqrt x} , dx leq int_0^{+infty}frac{1}{sqrt x },dx,$$ but this integral doesn't converge, so this way I can't say something about convergence of this integral.
convergence improper-integrals
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add a comment |
$begingroup$
Is
$$int_0^inftyfrac{sinln x}{sqrt x},dx$$
convergent? Absolutely convergent?
I found, that in fact it converges, but the only way I found is to say that $$int_0^{+infty}frac{sin ln x}{sqrt x} , dx leq int_0^{+infty}frac{1}{sqrt x },dx,$$ but this integral doesn't converge, so this way I can't say something about convergence of this integral.
convergence improper-integrals
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$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
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– tilper
May 10 '17 at 13:59
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@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
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– Eugene Korotkov
May 10 '17 at 14:01
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Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23
add a comment |
$begingroup$
Is
$$int_0^inftyfrac{sinln x}{sqrt x},dx$$
convergent? Absolutely convergent?
I found, that in fact it converges, but the only way I found is to say that $$int_0^{+infty}frac{sin ln x}{sqrt x} , dx leq int_0^{+infty}frac{1}{sqrt x },dx,$$ but this integral doesn't converge, so this way I can't say something about convergence of this integral.
convergence improper-integrals
$endgroup$
Is
$$int_0^inftyfrac{sinln x}{sqrt x},dx$$
convergent? Absolutely convergent?
I found, that in fact it converges, but the only way I found is to say that $$int_0^{+infty}frac{sin ln x}{sqrt x} , dx leq int_0^{+infty}frac{1}{sqrt x },dx,$$ but this integral doesn't converge, so this way I can't say something about convergence of this integral.
convergence improper-integrals
convergence improper-integrals
edited Dec 28 '18 at 8:26
Rócherz
2,7762721
2,7762721
asked May 10 '17 at 13:55
Eugene KorotkovEugene Korotkov
1689
1689
$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
$endgroup$
– tilper
May 10 '17 at 13:59
$begingroup$
@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
$endgroup$
– Eugene Korotkov
May 10 '17 at 14:01
$begingroup$
Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23
add a comment |
$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
$endgroup$
– tilper
May 10 '17 at 13:59
$begingroup$
@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
$endgroup$
– Eugene Korotkov
May 10 '17 at 14:01
$begingroup$
Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23
$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
$endgroup$
– tilper
May 10 '17 at 13:59
$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
$endgroup$
– tilper
May 10 '17 at 13:59
$begingroup$
@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
$endgroup$
– Eugene Korotkov
May 10 '17 at 14:01
$begingroup$
@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
$endgroup$
– Eugene Korotkov
May 10 '17 at 14:01
$begingroup$
Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23
$begingroup$
Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Use substitution $x=e^u$ to get
$$I=int_0^{+infty}frac{sinln x}{sqrt x},dx = int_{-infty}^{+infty}frac{sin u}{e^{u/2}}e^u,du = int_{-infty}^{+infty}e^{u/2}sin u,du$$
and then use partial integration:
begin{align}I &=int_{-infty}^{+infty}e^{x/2}sin x,dx \ &=left[begin{array}{ll} u = sin x,&du=cos x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2int_{-infty}^{+infty}e^{x/2}cos x,dx \
&= left[begin{array}{ll}u = cos x,&du=-sin x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2left(left.2e^{x/2}cos x,right|_{-infty}^{+infty} + 2int_{-infty}^{+infty}e^{x/2}sin x,dxright) \
&= left.2e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}-4Iend{align}
We conclude that $I = frac 25 left.e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}$ which doesn't converge.
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add a comment |
$begingroup$
Hint: For $n=1,2,dots,$ estimate the integral over $[e^{2pi n +pi/4},e^{2pi n +pi/2}].$ Note that $sin (ln x) ge 1/sqrt 2$ on this interval.
$endgroup$
add a comment |
$begingroup$
You could solve the indefinite integral,
begin{align*}
intfrac{sin(ln(x))}{sqrt x},dx &= intfrac{sqrt x cdotsin(ln(x))}{x},dx \ &= int e^{lnsqrt x}sin(ln x),d(ln x) \ &= int e^{frac12 ln x}sin(ln x),d(ln x) \ &= frac25sqrt xleft(2cos(ln x))-sin(ln x))right)\
end{align*}
and then evaluate the limits.
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Use substitution $x=e^u$ to get
$$I=int_0^{+infty}frac{sinln x}{sqrt x},dx = int_{-infty}^{+infty}frac{sin u}{e^{u/2}}e^u,du = int_{-infty}^{+infty}e^{u/2}sin u,du$$
and then use partial integration:
begin{align}I &=int_{-infty}^{+infty}e^{x/2}sin x,dx \ &=left[begin{array}{ll} u = sin x,&du=cos x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2int_{-infty}^{+infty}e^{x/2}cos x,dx \
&= left[begin{array}{ll}u = cos x,&du=-sin x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2left(left.2e^{x/2}cos x,right|_{-infty}^{+infty} + 2int_{-infty}^{+infty}e^{x/2}sin x,dxright) \
&= left.2e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}-4Iend{align}
We conclude that $I = frac 25 left.e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}$ which doesn't converge.
$endgroup$
add a comment |
$begingroup$
Use substitution $x=e^u$ to get
$$I=int_0^{+infty}frac{sinln x}{sqrt x},dx = int_{-infty}^{+infty}frac{sin u}{e^{u/2}}e^u,du = int_{-infty}^{+infty}e^{u/2}sin u,du$$
and then use partial integration:
begin{align}I &=int_{-infty}^{+infty}e^{x/2}sin x,dx \ &=left[begin{array}{ll} u = sin x,&du=cos x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2int_{-infty}^{+infty}e^{x/2}cos x,dx \
&= left[begin{array}{ll}u = cos x,&du=-sin x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2left(left.2e^{x/2}cos x,right|_{-infty}^{+infty} + 2int_{-infty}^{+infty}e^{x/2}sin x,dxright) \
&= left.2e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}-4Iend{align}
We conclude that $I = frac 25 left.e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}$ which doesn't converge.
$endgroup$
add a comment |
$begingroup$
Use substitution $x=e^u$ to get
$$I=int_0^{+infty}frac{sinln x}{sqrt x},dx = int_{-infty}^{+infty}frac{sin u}{e^{u/2}}e^u,du = int_{-infty}^{+infty}e^{u/2}sin u,du$$
and then use partial integration:
begin{align}I &=int_{-infty}^{+infty}e^{x/2}sin x,dx \ &=left[begin{array}{ll} u = sin x,&du=cos x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2int_{-infty}^{+infty}e^{x/2}cos x,dx \
&= left[begin{array}{ll}u = cos x,&du=-sin x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2left(left.2e^{x/2}cos x,right|_{-infty}^{+infty} + 2int_{-infty}^{+infty}e^{x/2}sin x,dxright) \
&= left.2e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}-4Iend{align}
We conclude that $I = frac 25 left.e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}$ which doesn't converge.
$endgroup$
Use substitution $x=e^u$ to get
$$I=int_0^{+infty}frac{sinln x}{sqrt x},dx = int_{-infty}^{+infty}frac{sin u}{e^{u/2}}e^u,du = int_{-infty}^{+infty}e^{u/2}sin u,du$$
and then use partial integration:
begin{align}I &=int_{-infty}^{+infty}e^{x/2}sin x,dx \ &=left[begin{array}{ll} u = sin x,&du=cos x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2int_{-infty}^{+infty}e^{x/2}cos x,dx \
&= left[begin{array}{ll}u = cos x,&du=-sin x\ v =2e^{x/2},&dv = e^{x/2}end{array}right] \
&= left.2e^{x/2}sin x,right|_{-infty}^{+infty} - 2left(left.2e^{x/2}cos x,right|_{-infty}^{+infty} + 2int_{-infty}^{+infty}e^{x/2}sin x,dxright) \
&= left.2e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}-4Iend{align}
We conclude that $I = frac 25 left.e^{x/2}(sin x-2cos x),right|_{-infty}^{+infty}$ which doesn't converge.
answered May 10 '17 at 14:58
EnnarEnnar
14.5k32443
14.5k32443
add a comment |
add a comment |
$begingroup$
Hint: For $n=1,2,dots,$ estimate the integral over $[e^{2pi n +pi/4},e^{2pi n +pi/2}].$ Note that $sin (ln x) ge 1/sqrt 2$ on this interval.
$endgroup$
add a comment |
$begingroup$
Hint: For $n=1,2,dots,$ estimate the integral over $[e^{2pi n +pi/4},e^{2pi n +pi/2}].$ Note that $sin (ln x) ge 1/sqrt 2$ on this interval.
$endgroup$
add a comment |
$begingroup$
Hint: For $n=1,2,dots,$ estimate the integral over $[e^{2pi n +pi/4},e^{2pi n +pi/2}].$ Note that $sin (ln x) ge 1/sqrt 2$ on this interval.
$endgroup$
Hint: For $n=1,2,dots,$ estimate the integral over $[e^{2pi n +pi/4},e^{2pi n +pi/2}].$ Note that $sin (ln x) ge 1/sqrt 2$ on this interval.
answered May 10 '17 at 14:22
zhw.zhw.
73.3k43175
73.3k43175
add a comment |
add a comment |
$begingroup$
You could solve the indefinite integral,
begin{align*}
intfrac{sin(ln(x))}{sqrt x},dx &= intfrac{sqrt x cdotsin(ln(x))}{x},dx \ &= int e^{lnsqrt x}sin(ln x),d(ln x) \ &= int e^{frac12 ln x}sin(ln x),d(ln x) \ &= frac25sqrt xleft(2cos(ln x))-sin(ln x))right)\
end{align*}
and then evaluate the limits.
$endgroup$
add a comment |
$begingroup$
You could solve the indefinite integral,
begin{align*}
intfrac{sin(ln(x))}{sqrt x},dx &= intfrac{sqrt x cdotsin(ln(x))}{x},dx \ &= int e^{lnsqrt x}sin(ln x),d(ln x) \ &= int e^{frac12 ln x}sin(ln x),d(ln x) \ &= frac25sqrt xleft(2cos(ln x))-sin(ln x))right)\
end{align*}
and then evaluate the limits.
$endgroup$
add a comment |
$begingroup$
You could solve the indefinite integral,
begin{align*}
intfrac{sin(ln(x))}{sqrt x},dx &= intfrac{sqrt x cdotsin(ln(x))}{x},dx \ &= int e^{lnsqrt x}sin(ln x),d(ln x) \ &= int e^{frac12 ln x}sin(ln x),d(ln x) \ &= frac25sqrt xleft(2cos(ln x))-sin(ln x))right)\
end{align*}
and then evaluate the limits.
$endgroup$
You could solve the indefinite integral,
begin{align*}
intfrac{sin(ln(x))}{sqrt x},dx &= intfrac{sqrt x cdotsin(ln(x))}{x},dx \ &= int e^{lnsqrt x}sin(ln x),d(ln x) \ &= int e^{frac12 ln x}sin(ln x),d(ln x) \ &= frac25sqrt xleft(2cos(ln x))-sin(ln x))right)\
end{align*}
and then evaluate the limits.
edited Dec 28 '18 at 8:31
Rócherz
2,7762721
2,7762721
answered May 10 '17 at 14:47
user409521
add a comment |
add a comment |
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$begingroup$
If $displaystyle int_0^{+infty} dfrac{sinln x}{sqrt x} , dx le int_0^{+infty} dfrac1{sqrt x}, dx$ and $displaystyleint_0^{+infty} dfrac1{sqrt x}, dx$ doesn't converge, then you can't conclude anything about $displaystyleint_0^{+infty} dfrac{sinln x}{sqrt x} , dx$. It sounds like you might know that, but you also said you found that the given integral does converge. How'd you determine that?
$endgroup$
– tilper
May 10 '17 at 13:59
$begingroup$
@tipler, yes, of course I understand this, but I didn't found any another ways, so, i'm trying to say, that I tried to use this method, but this method not right way
$endgroup$
– Eugene Korotkov
May 10 '17 at 14:01
$begingroup$
Why do you think the integral converges?
$endgroup$
– zhw.
May 10 '17 at 14:23