Problem 1.7 in G.Teschl ODE and Dynamical Systems asks me to transform the following differential eq

kolutastmr

kolutastmr

Answered question

2022-07-01

Problem 1.7 in G.Teschl ODE and Dynamical Systems asks me to transform the following differential equation into autonomous first-order system:
x ¨ = t sin ( x ˙ ) + x
Transforming the ODE to a system is in this case easy, but whats the usual technique to transform it to an AUTONOMOUS system?

Answer & Explanation

Bruno Dixon

Bruno Dixon

Beginner2022-07-02Added 14 answers

Given that
(1) x ¨ = t sin x ˙ + x ,
we may set
(2) y = x ˙ ,
then
(3) y ˙ = x ¨ ,
and we have
(4) y ˙ = t sin y + x ;
we also need to replace the independent variable t with yet a third (in addition to x and y) dependent variable we shall call z, which obeys
(5) z ˙ = 1 ;
this equation implies that
(6) z = t + c , c R ;
we need to ensure that c=0; this may be effected by setting an initial value for z, to wit:
(7) z ( t 0 ) = t 0 ;
then
(8) c = 0 ,
and
(9) z = t ;
thus the sought-for autonomous system may be written
(10) x ˙ = y ,
(11) y ˙ = z sin y + x ,
(12) z ˙ = 1 , z ( t 0 ) = t 0 ;
note we also need to specify x ( t 0 ) and y ( t 0 ) to obtain a unique solution.

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