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yifli
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I'm reading the differential equations chapter of Advanced Calculus by Loomis, and have some questions.
First it proved the following theorem:
Let A be and open subset of a Banach space W, let I be an open interval in R, and let F be a continuous mapping from I X A to W which has a continuous second partial differential. Then for any point [itex]<t_0, \alpha_0>[/itex] in I X A, from some neighborhood U of [itex]\alpha_0[/itex] and for any sufficiently small interval J containing [itex]t_0[/itex], there is a unique function f from J to U which is a solution of the differential equation passing through the point [itex]<t_0, \alpha_0>[/itex]
Then it states the following Lemma:
Let [itex]g_1[/itex] and [itex]g_2[/itex] be any two solutions of [itex]d\alpha/dt=F(t,\alpha)[/itex] through [itex]<t_0, \alpha_0>[/itex]. Then [itex]g_1(t)=g_2(t)[/itex] for all t in the intersection [itex]J=J_1\cap J_2[/itex] of their domains.
What is the above lemma useful for? The theorem says there is only one solution through [itex]<t_0, \alpha_0>[/itex], why does the lemma say "g1 and g2 be any two solutions"?
First it proved the following theorem:
Let A be and open subset of a Banach space W, let I be an open interval in R, and let F be a continuous mapping from I X A to W which has a continuous second partial differential. Then for any point [itex]<t_0, \alpha_0>[/itex] in I X A, from some neighborhood U of [itex]\alpha_0[/itex] and for any sufficiently small interval J containing [itex]t_0[/itex], there is a unique function f from J to U which is a solution of the differential equation passing through the point [itex]<t_0, \alpha_0>[/itex]
Then it states the following Lemma:
Let [itex]g_1[/itex] and [itex]g_2[/itex] be any two solutions of [itex]d\alpha/dt=F(t,\alpha)[/itex] through [itex]<t_0, \alpha_0>[/itex]. Then [itex]g_1(t)=g_2(t)[/itex] for all t in the intersection [itex]J=J_1\cap J_2[/itex] of their domains.
What is the above lemma useful for? The theorem says there is only one solution through [itex]<t_0, \alpha_0>[/itex], why does the lemma say "g1 and g2 be any two solutions"?