Inequality of ODE
$begingroup$
Suppose $u(t)>0$ satisfies following inequality, how do we solve $u$?
$$u^{'}(t)leq a cdot u^{frac{n+2}{n}}(t)$$
for $tgeq 0$. This seems a little different from Gronwall's inequality, how do we find inequality of $u(t)$, you can add some conditions to $u$ if you want. Thanks for any help.
ordinary-differential-equations
$endgroup$
add a comment |
$begingroup$
Suppose $u(t)>0$ satisfies following inequality, how do we solve $u$?
$$u^{'}(t)leq a cdot u^{frac{n+2}{n}}(t)$$
for $tgeq 0$. This seems a little different from Gronwall's inequality, how do we find inequality of $u(t)$, you can add some conditions to $u$ if you want. Thanks for any help.
ordinary-differential-equations
$endgroup$
4
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
2
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51
add a comment |
$begingroup$
Suppose $u(t)>0$ satisfies following inequality, how do we solve $u$?
$$u^{'}(t)leq a cdot u^{frac{n+2}{n}}(t)$$
for $tgeq 0$. This seems a little different from Gronwall's inequality, how do we find inequality of $u(t)$, you can add some conditions to $u$ if you want. Thanks for any help.
ordinary-differential-equations
$endgroup$
Suppose $u(t)>0$ satisfies following inequality, how do we solve $u$?
$$u^{'}(t)leq a cdot u^{frac{n+2}{n}}(t)$$
for $tgeq 0$. This seems a little different from Gronwall's inequality, how do we find inequality of $u(t)$, you can add some conditions to $u$ if you want. Thanks for any help.
ordinary-differential-equations
ordinary-differential-equations
asked Jan 3 at 7:24
STUDENTSTUDENT
1036
1036
4
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
2
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51
add a comment |
4
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
2
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51
4
4
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
2
2
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
We first take note of a few simple facts concerning $1 le n in Bbb N$:
$dfrac{n + 2}{n} = 1 + dfrac{2}{n}; tag 1$
set
$k = dfrac{2}{n}; tag 2$
then
$dfrac{n + 2}{n} = 1 + k; tag 3$
$u'(t) le au^{1 + k}; tag 3$
$u^{-k - 1} u'(t) le a; tag 4$
$(-dfrac{1}{k} u^{-k})' le a; tag 5$
we next integrate 'twixt $t_0$ and $t > t_0$:
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) = displaystyle int_{t_0}^t (-dfrac{1}{k} u^{-k}(s))' ; ds le a(t - t_0); tag 6$
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) le a(t - t_0); tag 7$
$-dfrac{1}{k}u^{-k}(t) le a(t - t_0) - dfrac{1}{k}u^{-k}(t_0) ; tag 8$
$u^{-k}(t) ge u^{-k}(t_0) -ka(t - t_0); tag 9$
since
$u(t) > 0, tag{10}$
it follows that the both sides of (9) are positive for $t$ sufficiently close to $t_0$, and always if $t > t_0$ provided $a le 0$; hence for such $t$,
$0 < u^k(t) le (u^{-k}(t_0) -ka(t - t_0))^{-1} = dfrac{1}{u^{-k}(t_0) -ka(t - t_0)} = dfrac{u^k(t_0) }{1 -ku^k(t_0)a(t - t_0)}, tag{11}$
whence
$0 < u(t) le ((u^{-k}(t_0) -ka(t - t_0))^{-1/k} = dfrac{1}{sqrt[k]{u^{-k}(t_0) -ka(t - t_0)}} = dfrac{u(t_0) }{sqrt[k]{1 - ku^k(t_0)a(t - t_0)}}. tag{12}$
Inspection of (12) reveals that
$displaystyle lim_{t to infty} u(t) = 0, ; a < 0, tag{13}$
whereas the limit, if one exists, is not determined from (12) if $a ge 0$. In fact, a singularity is encountered as
$t to t_0 + dfrac{1}{k u^k(t_0)a} tag{14}$
from below. The situation as $t to -infty$ is similar, with the cases $a < 0$, $a ge 0$ interchanged.
$endgroup$
add a comment |
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1 Answer
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1 Answer
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$begingroup$
We first take note of a few simple facts concerning $1 le n in Bbb N$:
$dfrac{n + 2}{n} = 1 + dfrac{2}{n}; tag 1$
set
$k = dfrac{2}{n}; tag 2$
then
$dfrac{n + 2}{n} = 1 + k; tag 3$
$u'(t) le au^{1 + k}; tag 3$
$u^{-k - 1} u'(t) le a; tag 4$
$(-dfrac{1}{k} u^{-k})' le a; tag 5$
we next integrate 'twixt $t_0$ and $t > t_0$:
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) = displaystyle int_{t_0}^t (-dfrac{1}{k} u^{-k}(s))' ; ds le a(t - t_0); tag 6$
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) le a(t - t_0); tag 7$
$-dfrac{1}{k}u^{-k}(t) le a(t - t_0) - dfrac{1}{k}u^{-k}(t_0) ; tag 8$
$u^{-k}(t) ge u^{-k}(t_0) -ka(t - t_0); tag 9$
since
$u(t) > 0, tag{10}$
it follows that the both sides of (9) are positive for $t$ sufficiently close to $t_0$, and always if $t > t_0$ provided $a le 0$; hence for such $t$,
$0 < u^k(t) le (u^{-k}(t_0) -ka(t - t_0))^{-1} = dfrac{1}{u^{-k}(t_0) -ka(t - t_0)} = dfrac{u^k(t_0) }{1 -ku^k(t_0)a(t - t_0)}, tag{11}$
whence
$0 < u(t) le ((u^{-k}(t_0) -ka(t - t_0))^{-1/k} = dfrac{1}{sqrt[k]{u^{-k}(t_0) -ka(t - t_0)}} = dfrac{u(t_0) }{sqrt[k]{1 - ku^k(t_0)a(t - t_0)}}. tag{12}$
Inspection of (12) reveals that
$displaystyle lim_{t to infty} u(t) = 0, ; a < 0, tag{13}$
whereas the limit, if one exists, is not determined from (12) if $a ge 0$. In fact, a singularity is encountered as
$t to t_0 + dfrac{1}{k u^k(t_0)a} tag{14}$
from below. The situation as $t to -infty$ is similar, with the cases $a < 0$, $a ge 0$ interchanged.
$endgroup$
add a comment |
$begingroup$
We first take note of a few simple facts concerning $1 le n in Bbb N$:
$dfrac{n + 2}{n} = 1 + dfrac{2}{n}; tag 1$
set
$k = dfrac{2}{n}; tag 2$
then
$dfrac{n + 2}{n} = 1 + k; tag 3$
$u'(t) le au^{1 + k}; tag 3$
$u^{-k - 1} u'(t) le a; tag 4$
$(-dfrac{1}{k} u^{-k})' le a; tag 5$
we next integrate 'twixt $t_0$ and $t > t_0$:
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) = displaystyle int_{t_0}^t (-dfrac{1}{k} u^{-k}(s))' ; ds le a(t - t_0); tag 6$
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) le a(t - t_0); tag 7$
$-dfrac{1}{k}u^{-k}(t) le a(t - t_0) - dfrac{1}{k}u^{-k}(t_0) ; tag 8$
$u^{-k}(t) ge u^{-k}(t_0) -ka(t - t_0); tag 9$
since
$u(t) > 0, tag{10}$
it follows that the both sides of (9) are positive for $t$ sufficiently close to $t_0$, and always if $t > t_0$ provided $a le 0$; hence for such $t$,
$0 < u^k(t) le (u^{-k}(t_0) -ka(t - t_0))^{-1} = dfrac{1}{u^{-k}(t_0) -ka(t - t_0)} = dfrac{u^k(t_0) }{1 -ku^k(t_0)a(t - t_0)}, tag{11}$
whence
$0 < u(t) le ((u^{-k}(t_0) -ka(t - t_0))^{-1/k} = dfrac{1}{sqrt[k]{u^{-k}(t_0) -ka(t - t_0)}} = dfrac{u(t_0) }{sqrt[k]{1 - ku^k(t_0)a(t - t_0)}}. tag{12}$
Inspection of (12) reveals that
$displaystyle lim_{t to infty} u(t) = 0, ; a < 0, tag{13}$
whereas the limit, if one exists, is not determined from (12) if $a ge 0$. In fact, a singularity is encountered as
$t to t_0 + dfrac{1}{k u^k(t_0)a} tag{14}$
from below. The situation as $t to -infty$ is similar, with the cases $a < 0$, $a ge 0$ interchanged.
$endgroup$
add a comment |
$begingroup$
We first take note of a few simple facts concerning $1 le n in Bbb N$:
$dfrac{n + 2}{n} = 1 + dfrac{2}{n}; tag 1$
set
$k = dfrac{2}{n}; tag 2$
then
$dfrac{n + 2}{n} = 1 + k; tag 3$
$u'(t) le au^{1 + k}; tag 3$
$u^{-k - 1} u'(t) le a; tag 4$
$(-dfrac{1}{k} u^{-k})' le a; tag 5$
we next integrate 'twixt $t_0$ and $t > t_0$:
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) = displaystyle int_{t_0}^t (-dfrac{1}{k} u^{-k}(s))' ; ds le a(t - t_0); tag 6$
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) le a(t - t_0); tag 7$
$-dfrac{1}{k}u^{-k}(t) le a(t - t_0) - dfrac{1}{k}u^{-k}(t_0) ; tag 8$
$u^{-k}(t) ge u^{-k}(t_0) -ka(t - t_0); tag 9$
since
$u(t) > 0, tag{10}$
it follows that the both sides of (9) are positive for $t$ sufficiently close to $t_0$, and always if $t > t_0$ provided $a le 0$; hence for such $t$,
$0 < u^k(t) le (u^{-k}(t_0) -ka(t - t_0))^{-1} = dfrac{1}{u^{-k}(t_0) -ka(t - t_0)} = dfrac{u^k(t_0) }{1 -ku^k(t_0)a(t - t_0)}, tag{11}$
whence
$0 < u(t) le ((u^{-k}(t_0) -ka(t - t_0))^{-1/k} = dfrac{1}{sqrt[k]{u^{-k}(t_0) -ka(t - t_0)}} = dfrac{u(t_0) }{sqrt[k]{1 - ku^k(t_0)a(t - t_0)}}. tag{12}$
Inspection of (12) reveals that
$displaystyle lim_{t to infty} u(t) = 0, ; a < 0, tag{13}$
whereas the limit, if one exists, is not determined from (12) if $a ge 0$. In fact, a singularity is encountered as
$t to t_0 + dfrac{1}{k u^k(t_0)a} tag{14}$
from below. The situation as $t to -infty$ is similar, with the cases $a < 0$, $a ge 0$ interchanged.
$endgroup$
We first take note of a few simple facts concerning $1 le n in Bbb N$:
$dfrac{n + 2}{n} = 1 + dfrac{2}{n}; tag 1$
set
$k = dfrac{2}{n}; tag 2$
then
$dfrac{n + 2}{n} = 1 + k; tag 3$
$u'(t) le au^{1 + k}; tag 3$
$u^{-k - 1} u'(t) le a; tag 4$
$(-dfrac{1}{k} u^{-k})' le a; tag 5$
we next integrate 'twixt $t_0$ and $t > t_0$:
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) = displaystyle int_{t_0}^t (-dfrac{1}{k} u^{-k}(s))' ; ds le a(t - t_0); tag 6$
$-dfrac{1}{k}u^{-k}(t) + dfrac{1}{k}u^{-k}(t_0) le a(t - t_0); tag 7$
$-dfrac{1}{k}u^{-k}(t) le a(t - t_0) - dfrac{1}{k}u^{-k}(t_0) ; tag 8$
$u^{-k}(t) ge u^{-k}(t_0) -ka(t - t_0); tag 9$
since
$u(t) > 0, tag{10}$
it follows that the both sides of (9) are positive for $t$ sufficiently close to $t_0$, and always if $t > t_0$ provided $a le 0$; hence for such $t$,
$0 < u^k(t) le (u^{-k}(t_0) -ka(t - t_0))^{-1} = dfrac{1}{u^{-k}(t_0) -ka(t - t_0)} = dfrac{u^k(t_0) }{1 -ku^k(t_0)a(t - t_0)}, tag{11}$
whence
$0 < u(t) le ((u^{-k}(t_0) -ka(t - t_0))^{-1/k} = dfrac{1}{sqrt[k]{u^{-k}(t_0) -ka(t - t_0)}} = dfrac{u(t_0) }{sqrt[k]{1 - ku^k(t_0)a(t - t_0)}}. tag{12}$
Inspection of (12) reveals that
$displaystyle lim_{t to infty} u(t) = 0, ; a < 0, tag{13}$
whereas the limit, if one exists, is not determined from (12) if $a ge 0$. In fact, a singularity is encountered as
$t to t_0 + dfrac{1}{k u^k(t_0)a} tag{14}$
from below. The situation as $t to -infty$ is similar, with the cases $a < 0$, $a ge 0$ interchanged.
edited Jan 4 at 22:41
answered Jan 3 at 22:41
Robert LewisRobert Lewis
48.6k23167
48.6k23167
add a comment |
add a comment |
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4
$begingroup$
$frac{du}{u^{frac{n+2}{n}}}le a dtto frac 12 n u^{-frac 2n}le a t + C_0$
$endgroup$
– Cesareo
Jan 3 at 10:18
2
$begingroup$
@Cesareo: There is a minus sign missing in $-frac12nu(t)^{-frac2n}le at-frac12nu_0^{-frac2n}$, so that then $$u(t)le frac{u_0}{(1-frac2nu_0^{frac2n}at)^{frac{n}2}}.$$
$endgroup$
– LutzL
Jan 3 at 23:15
$begingroup$
@LutzL Thanks for the hint.
$endgroup$
– Cesareo
Jan 3 at 23:38
$begingroup$
This bound is only valid until its singularity, even if there is a real branch of the root after that, it does no longer relate to $u$.
$endgroup$
– LutzL
Jan 3 at 23:51